Theo

Theo is a user of the Persona Compendium and a self-described "governor of power". What this means is never fully described, but he is at least demonstrated to be an extremely powerful individual, on par with various in-series gods and similar: it can be presumed that his level of strength is similar to his sister's, who has been seen to obliterate the incarnation of a force of nature in one blow.

In theory, it is very likely he could access any Persona in existence as of his featured game. However, he has only been actively seen to use a small selection. He favours Personas with an Abrahamic religious theme, angels and devils.

In P3 he uses Uriel, Gabriel, Michael, Raphael, Sandalphon, Lilith, Abaddon, Beelzebub and Metatron. In PQ he uses Beelzebub, Gabriel, Sandalphon and Lilith from that list, and adds Satan to the lineup. ... Yeah.

Between them, his P3 set provides coverage for all elements found in his game's attacks (wind, fire, ice, lightning, light, dark), a healing skill, a range of status effects, and the "9999 HP Megidolaon" super attack. In PQ he uses what is more of a support set of moves.

( Persona stats, based on P3P )

(Using the "usual" names for the P3 protags for the sake of this post, but cool with any in play:)

* Presumably ~18/19 for P3/Q canon point, if Velvet Family age/mature the same way as humans? Going from the fact that while he's younger than Elizabeth, she's still young enough to be reasonably dating Minato. On top of that, Junpei and potentially Minato both think he looks a lot like Kanji - a fifteen-year-old who looks a bit older than his age, though it was at least recognised that Theo was the older of the two.

* Unsure where the Velvet Family actually originate. Vague headcanon is that they're created artificially (not sure if directly by Philemon, or by Igor or some other individual?) as conduits for/to be able to handle the sort of power and knowledge that their duties require - though whether that's as babies (see above) or as more mature beings, very uncertain. Though the siblings being too young to be attendants during P1/2 would explain their absence somewhat? Or they could just have been busy.
In other words, there's some massive handwaving going on here.

* Willing to play him to match either P3 protag's story. If he comes from a timeline where Minako wasn't around (canon PQ, etc) or where she had Elizabeth attending, he was presumably just in the background taking care of whatever other duties the Velvet Room requires during P3, in the same way Margaret presumably was. Possibly research into myths and Personas and guests' backgrounds, archiving records and the likes, or being bossed into cleaning up by his sisters.

* Nameless tried to teach him to play piano. It wasn't entirely successful.

* Perhaps even in Minato's timeline, he still dreams of a red-headed girl and wonders where the image of a long-haired Orpheus in his book got there...

* There is a lot more to the Velvet Room than what guests see. (Otherwise where would he be asking to put the crane games?)

* Possibly Hermit Arcana? Star?