Trigger warnings: DID, trauma, dissociation, brief mentions/implications of great harm to children (including but not limited to abuse), miscarriage.

Because DID generally requires chronic trauma in childhood (as opposed to one singular incident), this headcanon does rely on some canon divergence. Primarily, I've drawn on the fact that the Wayne family (while not mobsters themselves), have often been depicted with connections to major crime families (such as Thomas having a friendship with the Falcones), multiple of Bruce's childhood friends/acquaintances grow up to be supervillains and those issues started in childhood (Roman Sionis, Thomas Elliot being major examples), and the fact that while loving parents, Thomas and Martha are often depicted as incredibly busy people, giving young Bruce a less secure attachment to his caregivers before their death. After their death, Alfred functions as primary caregiver, but this relationship is often depicted with some role confusion. Furthermore, there are stories of people trying to use the death of Thomas and Martha to take advantage of young Bruce, plus in-universe, their deaths have been highly sensationalized, which is not an ideal environment for a grieving child.

So in addition to witnessing his parents' murder, I am extrapolating from various source material: bullying and harassment from people such as Thomas Elliot and Roman Sionis, witnessing mob violence from an early age (such as the Falcones bringing someone injured to the Wayne Manor), non-ideal attachments with caregivers, people generally trying to use the death of his parents against Bruce in childhood, and constant reminders of the event from the celebrity status the Wayne family has in Gotham.

Please note: Becoming a vigilante was a group decision among the system because, while they are all different, they ultimately all believe in protecting the innocent and keeping people safe from violence. There is some strife between some of the people within the system, but it should be remembered that even difficult alters serve a function within a system and should not be treated as "evil".

Wayne

Name: (Bruce) Wayne. Answers to Bruce and Wayne from other people, but headmates and people who know that he has DID call him Wayne
Age: Ages with the body
Gender: Cis man
Pronouns: He/Him/His
Sexuality: Pan & demi
Inner world description: Looks like the body, typically dresses in classy suits, dark blues and black. Has private quarters adjacent to the throne room, so he can be on hand for discussion or intervention with people fronting.
Personality: Dour and reserved, but inquisitive and curious. Likes reading, mysteries, and is very compassionate, albeit shy. Remembers his parents’ death, but not much of the other childhood trauma.
Relationship to headmates: As the host, most of the alters defer to him when there’s conflict over decisions that affect the whole system. He runs most of the day-to-day life and coordinates with Batman to keep vigilante operations running smoothly.
Relationship to family: Gets along well with Alfred, somewhat regards him as a father, but doesn’t like to say so. Father to the batkids (Dick, Jason, Tim, Damian, Cass).
Other relationships: Has dated Selina. Gets along with Talia, but isn’t dating her. A few members of the Justice League know he has DID, but not all of them, not even all the ones who know Batman’s secret identity. Was best friends with Harvey Dent, but never told him that he has DID or about Batman.

Batman

Name: Batman
Age: Ages with the body
Gender: Agender
Pronouns: He/Him
Sexuality: Pan & demi
Inner world description: Appears as a humanoid creature, cloaked in shadows. Originally looked very similar to Zorro, but over time developed an appearance more akin to the Batman costume, but the features are part of his body, rather than clothing. He has no set residence within the inner world, flying from the parapets, the bell tower, the caves and catacombs, and around the inside of the castle.
Personality: Secretive, but caring. A strong sense of justice and a desire to protect the innocent, as well as the system. Compassionate and thoughtful, but reluctant to show it for fear of this kindness being taken advantage of or used to hurt the system. Holds some childhood trauma, such as getting lost in the caves and woods around the manor as a small child. Holds most of the trauma that the system goes through during vigilantism, though some more extreme events get diverted to others (Detective, Knight, and Gargoyle).
Relationship to headmates: While Wayne can override him, Batman is primarily viewed as the “boss” of the system. He’s in charge of keeping them safe and he comes up with a lot of rules for the system to follow to that end. He takes care of Gargoyle as well. The only alter who doesn’t get along well with him is Detective, who is also trying to protect the system, but has different ideas about what will protect them. Usually, Batman and Detective are able to cooperate, but sometimes there is extreme friction.
Relationship to family: Considers Alfred his father, but avoids saying so. Also is the father of the batkids (Dick, Jason, Tim, Damian, and Cass).
Other relationships: Has an on/off relationship to Selina. Close friends with some of the JL, but very reluctant to tell them that he is an alter. Was friends with Harvey, separately from Wayne. Briefly dated Talia, but broke it off as Detective fell in love with her and Batman was put off by Ra’s.

Bruce

Name: Bruce (Wayne). Answers to both Bruce and Wayne from outsiders, but headmates call him Bruce
Age: Aged with the body up to about 25, now slides between mid-20s and mid-30s
Gender: Cis man
Pronouns: He/Him/His
Sexuality: Pan as heck
Inner world description: Looks like a younger version of Wayne and tends to dress in more colorful suits and sporty attire. He lives inside the castle, in a lavish suite with a balcony near the throne room.
Personality: Friendly, outgoing, and a bit of a player. He comes across as stupid, though that’s not entirely accurate, his intelligence is just strictly interpersonal and not very academic at all. He has a lot of physical relationships, but is very upfront that they’re short-term and very respectful of his partners. Because he keeps people emotionally distant, he allows the system to have physical intimacy without feeling at risk of their secrets being exposed. Bruce holds no traumatic memories for the system.
Relationship to headmates: Gets along well with Wayne. A lot of the other alters find him a bit annoying because he’s too chipper or buffoonish to match most of them, but it’s the sort of annoyance one might have for a sibling, rather than outright dislike.
Relationship to family: Views Alfred more like a close friend/employee than a father or caretaker. Likes the batkids well enough, but doesn’t feel like he’s ready for fatherhood, so he sees them more like younger siblings.
Other relationships: Not a whole lot of longterm relationships. Generally relationships that seem like they’ll go that direction, Wayne will have them instead.

Arthur

Name: Arthur
Age: About 20
Gender: Cis man
Pronouns: He/Him
Sexuality: Doesn’t really think about it
Inner world description: Thin, delicate features. Looks much more like Martha than Thomas. Typically dresses nicely, in warm colors, wears a kippah. He lives in the library.
Personality: Soft, caring, and earnest. Most likely to ask for help from people outside the system, even if it means disclosing a few secrets. He doesn’t hold trauma, but instead remembers his childhood as idealistic and has a strong desire to make his parents proud. He was the one who wanted to go to med school, but it eventually became too taxing on him and Wayne to manage. Over time, Arthur fronted much less as the system began to value secrecy for their safety, but he never became fully dormant.
Relationship to headmates: Gets along quite well with all except Batman and Detective, who think his tendency to be open is a threat to overall safety. Batman doesn’t dislike him, but thinks he’s foolish with how much he shares. Detective dislikes him a little.
Relationship to family: Doesn’t think of Alfred as a father because he has so much of his identity attached to his parents, but does view Alfred as the system’s primary caretaker and will go to him with problems. Because he stopped aging around 20, he sees himself more like a brother to the batkids, though not all of them have met him because he so rarely fronts.
Other relationships: None

Samson

Name: Samson
Age: Early teens
Gender: Neutral
Pronouns: He/Him
Sexuality: None
Inner world description: Looks very similar to young Bruce, but with longer hair and a stockier build. He resides in the catacombs now, resting.
Personality: Quiet and patient. Samson holds most of the childhood trauma for the system, such as bullying from Tommy Elliot and Roman Sionis, keeping the memories locked away. He got his name from enduring a lot of punishment while waiting for a chance to get away. Around age 14, Samson became dormant as he was no longer actively needed, but the memories stayed with him, resulting in the system not having to recall most of the trauma from childhood.
Relationship to headmates: Doesn’t really have one. Most were unaware of each other while Samson was active and now Samson is dormant, so they just let him be.
Relationship to family: Also none. He had a couple of conversations with Alfred and Leslie, but then fell dormant.
Other relationships: Besides having been harassed and bullied by Tommy and Roman, he doesn’t really have any relationships.

Detective

Name: Detective (or Beloved, if you’re Talia)
Age: Views himself as ageless
Gender: Non-binary man
Pronouns: He/Him/His
Sexuality: Gray-aromantic
Inner world description: Narrower features than the body, hair slicked back to keep it out of his face. Tall, lean, typically dressed in martial arts gear. He resides in the catacombs, where he has a Lazarus pit that he protects from being misused. He has a British accent, but doesn’t have any particular nationality.
Personality: He comes across as cold and calculating, though he has a lot of quiet rage at injustice. He has more of a “do what must be done” approach to vigilantism than Batman, though he is still opposed to killing outright. He will, on rare occasions, allow people to die if he deems them sufficiently terrible. He is generally more willing to take morally questionable approaches for the greater good, but does have his limits. He holds a lot of trauma from training with the League of Shadows, having to face no-win scenarios, and seeing exceptionally evil things done, especially harm done to children. He has the benefit of being able to understand evil people without condoning their actions.
Relationship to headmates: Very rocky. Wayne makes an effort not to villainize Detective too much, but Detective is the most likely of the alters to go against group decisions. He can take on a somewhat persecutory role with Batman, pushing him past his limits when Batman is struggling. Over time, he has become more cooperative with the others, especially after pushing Batman too hard resulted in a lot of harm being done to the system, but he still steps in when Batman is overwhelmed.
Relationship to family: Mixed reviews! Detective, having developed when Wayne was abroad, has little connection to Alfred, and is therefore more likely to disrespect Alfred. After clashing a lot with Dick over morals, Detective interacted less with Jason and, carrying the trauma of Jason’s death, even less with Tim. He views Damian as his son and somewhat grudgingly shares him with Wayne and Batman.
Other relationships: Very much in love with Talia, even marrying her and having a child with her—though for a long time he thought that child had been lost. He’s had no other romantic relationships. While he was initially more willing to hear Ra’s out, over time his relationship with Ra’s soured because Ra’s is way too down with murder for even Detective. He tends to have a much more antagonistic relationship to the rogues because he’s more willing to use excessive force than Batman.

Myst

Name: Myst(ery)
Age: Rude to ask!
Gender: Genderqueer
Pronouns: They/Them
Sexuality: Pan and very much would like to have dates pls
Inner world description: Much more lean and flexible than Wayne, very androgynous appearance with long curly hair. Dresses in dark blues, purples, and sparkly blacks, sometimes with peacock feathers and galaxy patterns. Outfits range from catsuits to tuxes, like a magician. They live in the ballroom. They’re Italian and have an accent.
Personality: Unless they know someone well, they don’t talk very much, but in facial expression and physicality, they’re much more expressive than Batman. They’re a thrill seeker and escape artist who loves to put on performances. They live for theatrics and drama and delight in risks because they’re always confident they can get out of them. They don’t hold any trauma for the system and because they enjoy dangerous situations, they’ve allowed the system to learn to get out of death traps and such without having to further traumatize the system. They have a more detailed background life that deviates drastically from the history of the body than any of the other alters.
Relationship to headmates: Well-liked and appreciated. They don’t interact within the inner world with most of the other alters besides Batman, but none of them have ever had any reason to dislike Myst. Batman and Myst are very good at working together when needed, able to switch off as the situation demands it or co-front to get through death traps together. Myst thinks Batman can be over-protective, but appreciates him nonetheless.
Relationship to family: Loves to tease Alfred. Like Detective, they came about while Wayne was away from home, but they have a more friendly rapport with Alfred. Myst is more of a mentor than a parental figure to the batkids, helping to teach some of their tricks to the Robins. Especially fond of Dick and bonds with him over his background as a performer, but their less serious approach to vigilantism is quickly outgrown by the Robins.
Other relationships: Very fond of Giovanni and Zatanna Zatara. More likely to try to reach out to some of the rogues, though for the most part holds back so as to keep the system a secret.

Matches Malone

Name: Matches Malone
Age: A little older than the body
Gender: Cis man
Pronouns: He/Him
Sexuality: Gay
Inner world description: A world worn, humble looking fellow with a slightly graying mustache. Tends to wear a trench coat and a cap. Usually smoking in the inner world, but for the health of the body, has to quit that on the outside. Has a very thick Jersey accent. Lives in a guest room of the castle, though because he has a permanent residence, “guest” room is a bit of a misnomer.
Personality: A chill dude, unless you start shit with him. Matches has the memories of a low-ranking henchman, specializing in arson. He didn’t want to become a criminal, he just found his options severely limited. Matches holds empathy towards people who got stuck in a cycle of crime and trauma from interacting too much with Gotham’s underworld, as well as unintended deaths and collateral damage.
Relationship to headmates: Initially, Matches was met with confusion and wariness from the rest of the system, being both the first alter to emerge after the vigilantism started and being the first (and only) alter with explicit criminal memories. After some effort, however, they were able to work out an arrangement and Matches started helping out as an undercover informant. He still occasionally clashes with Batman and Detective, but because Matches has a laidback nature, it’s hard to dislike him for long, and Wayne has come to appreciate the internal check as a constant reminder of the humanity of criminals.
Relationship to family: Doesn’t consider them his family, but Matches is very easygoing and affable with Alfred and the Batfam.
Other relationships: Pals with Harley Quinn.

The (Fallen) Knight

Name: The (Fallen) Knight; usually just called the Knight because it’s less upsetting
Age: None
Gender: Neutral
Pronouns: He/Him
Sexuality: None
Inner world description: A knight in broken, tarnished armor, dried blood causing the metal to rust. He has trouble walking and standing, lingering pain from the injuries that “killed” him. When he speaks, his voice is strained and raspy. He kneels, leaning on his sword, outside the entrance to the catacombs.
Personality: A somber, defeated man. The Knight holds extreme trauma from near-death experiences, such as Bane breaking his back. He is dead, but lingers on to carry this weight and remind the others to not make the same mistakes.
Relationship to headmates: The Knight is treated with reverence and respect, especially from Detective, who sees to his needs as he stays outside the catacombs. He doesn’t speak much to them.
Relationship to family: He very rarely fronts, so he doesn’t have much of one.
Other relationships: Bane don’t interact.

Gargoyle

Name: The Gargoyle
Age: ????
Gender: ????
Pronouns: He/Him
Sexuality: ????
Inner world description: He looks like bat-themed stone gargoyle that’s come to life because that’s what he is. He lives on the parapets of the castle.
Personality: Very simplistic, due to his unusual origin and fragmentary nature. He guards the system from some nebulous fear, sitting vigil on the roof of the castle until he has to go to the front. When fronting, under the influence of Scarecrow’s fear toxin, his only goals are to capture Scarecrow if possible and get back to Alfred. He holds trauma specific to being repeatedly hit with fear toxin, but no other memories, keeping the secrets of the system safe from being released even while drugged.
Relationship to headmates: Somewhat viewed as the baby of the family. The alters are friendly with him on the inner world, though they don’t have much cause to interact with him. He’s comforting to have around as a protective ward from bad things and Gargoyle enjoys sitting his protective vigil because that’s what Gargoyles do.
Relationship to family: Protective of Robins, though he has trouble telling them apart because of the amnesia walls. Feels safest around Alfred, who is the one best able to calm him down enough to let another alter come to the front.
Other relationships: Fights Scarecrow.