Theodore, known as Teodor in the Japanese version, is a non-player character from Persona 3 Portable. He can be chosen by the player to serve as Igor's assistant in the Velvet Room if the female protagonist is chosen. He is the younger brother of Elizabeth and Margaret.
Theo has a similar appearance of Kanji Tatsumi, but with white hair and yellow eyes. He wears a black-collared and long-sleeved shirt under a blue suit complemented with a blue necktie. He also wears black pants with blue lines running through the sides, black shoes, and a hat similar to a flight attendant or even a bell boy. He also carries the Persona Compendium.
Theodore is a polite and serious young man. Despite his seemingly taciturn disposition, he is actually quite a show-off to the protagonist whenever he requests her to escort her in the outside world and, through this attitude, his naïveté greatly shows, usually with humorous repercussions.
Though nothing is shown of Theodore's interactions with his sisters in the game, some of his quests briefly hint of his relationship with them. Elizabeth's presence (or even just a thought) is implied to make him wary as Request #28 (which gives the task of finding a drink of Elizabeth's namesake), he states that the "Queen Elizabeth" gives him a chill down his spine leading to his decision of not drinking it. Also his naïve and vulnerable nature is hinted more during Request #63 when he muttered to himself of the time when "they" (hinting to Margaret and Elizabeth) fed him dog food (during which he had no knowledge of).
Like his sister to the male protagonist, Theo develops feelings for the female protagonist as they spend time together around Port Island. The result, though, is tragic with him breaking everything off, Theo's duties as the Velvet Room's attendant his reasons.
When Theo challenges the protagonist, his politeness is still shown, prompting the player to fight as if she wants to kill him. He also apologizes to her when he attacks and lands a hit. But even through his politeness, he won't think twice in using Megidolaon when certain easy to achieve requirements are met.
Theo can be found when the player pursues the female protagonist path when the appropriate choice is made. Aside from his usual requests, Theo will also tell the protagonist when there is a person who is stranded in Tartarus within a set range of floors. He can replace Elizabeth as Igor's assistant. He serves as one of the ultimate bosses for the female protagonist's path, the other being Margaret.
Some of his requests will require the female protagonist to take him to specific locations in the real world, considered dates. It is unknown whether or not this can be considered canon. Throughout his dates, he proves that he is very proud and modest, pretending to know everything there is to know about everything. He can determine the secret ingredient in takoyaki just by the smell of it, and can tell the temperature of water just by feeling it. He is not as humorous as Elizabeth, and most of his humor stems from his seriousness about his knowledge. He repeatedly asks Igor to install crane games and escalators, et cetera, into the Velvet Room, but always fails.
Theodore can only be chosen and subsequently fought by the female protagonist in Persona 3 Portable. Theodore's Personas are different than Elizabeth's during the optional fight, solely using Abrahamic Personas, but the skills used are entirely identical.
Like the battle with Elizabeth, he uses a set pattern of Personas that he will summon during the battle. The order of the pattern is: Uriel, Gabriel, Michael, Raphael, Sandalphon, Lilith, Abaddon, and Beelzebub. Note that Theo uses Personas that are both Devils and Angels, the first four consisting of prominent angels and the latter three being prominent devils.
He is able to tell an object's temperature simply by feeling it.
He acts very stoically and proud, assuming that he knows everything, proven in the Paulownia Mall date.
He receives a takoyaki cell phone strap from the takoyaki cook during the Iwatodai Station date. He refuses to let the protagonist have it, as it is "her fault for making him obtain something precious to him."
He does not know the difference between a statement and a metaphor.
His diminutive "Theo" is a pun for the Japanese word "hand" (手お, teo) which he says it to ask the Female Protagonist's hand for showing his courtesy.
Oddly enough before fighting him, Fuuka will scan Theo saying "she", like the battle with Elizabeth.
He may be either named after the character Dr. Theodore Bohmer from Ghost of Frankenstein, or named after Theodor Richard Edward con Holst, the first illustrator of the novel.
During one of the quests it is implied that Margaret and Elizabeth once fed him dog food.
One of Elizabeth's battle quotes in Persona 4 Arena states that she, like Margaret, also scolds Theodore.
If the female protagonist equips Mara, then Theo will remark it is unladylike.
During one of his dates, he claims that he's an "avatar of power".
Theodore technically shares the same name with Teddie from Persona 4 as Ted is short for Theodore and Teddie is a somewhat informal name for Ted.
After completing all his 'Show me' quests, when the player talks to him in the quest/compendium screen, scroll down and there will be a new option labelled "Ask out".