Theodore is a non player character from Persona 3 Portable. He is the younger brother of Elizabeth and Margaret, and the older brother of Lavenza. He can be chosen by the player between him or Elizabeth to serve as Igor's assistant in the Velvet Room if the female protagonist is chosen.
It is impossible for him to be chosen by the male protagonist. Persona 3 Portable: Non. Persona's Velvet Room attendants have primarily been female, but there is one major reason Persona 6 may feature a male instead.
A full look at the new Velvet Room Costumes : r/PERSoNA
The names of the Velvet Room attendants are taken from the novel Frankenstein (except Theodore, whose name comes from the film The Ghost of Frankenstein). Almost all the female names that appear in the novel have already been used, only Agatha and Safie remain. Social Links with male characters are added, with some having the option of being romantic, although there's an option to stay platonic.
You have the choice of a new male Velvet Room attendant named Theodore. Community content is available under CC. Theodore only exist and added for the female route when Atlus was making Portable.
Velvet Room Attendant Goro | Persona 5, Room attendant, Persona
At the end of the day, it's pointless for Theodore to be Male Protagonist's Velvet Room attendant one way or another because it would be totally non-canon since Elizabeth is officially Male Protagonist's Velvet Room attendant. Every resident in the Room, Morgana included, is named after a character in Frankenstein. And Victor Frankenstein himself has yet to be represented as Theo is the only male attendant.
So Victor will possibly be Persona 6′s attendant. And his role within the Velvet Room's over all Hotel theme? Possibly a Chef. Uncover connections between Velvet Room members and classic works such as Frankenstein.
Velvet Room Attendants and their namesakes : r/PERSoNA
Explore fan theories on potential names for upcoming Velvet Room characters. Persona 6 Expectations Users speculate on the potential choices for the attendant names in Persona 6, drawing connections to literary figures such as Alphonse and Mary. Another idea, a young male attendant named William who ends up dying for story reasons (like by a persona/shadow based on the monster that MC unintentional summons and goes out of control).
Replaced by another older attendant named Ernest when that happens. They make references to Victor from Devil Summoner throughout the game in small ways too.