Matsu is a new agent partnered with Agent Reiar/Rei AR.
Matsu has bristly black hair and black eyes (dark brown, if one wants to be obsessive about it). He wears a black unbuttoned short-sleeve button-up shirt over a whitish shirt and white jeans.
Short version: if Matsu was a Dungeons and Dragons character, he would be Chaotic Evil.
Long version: Matsu generally regards rules as something invented solely to tick people off, and ignores them. Unfortunately, these typically tend to be things such as "Thou shalt NOT shatter a gigantic mirror floating above a roomful of generics." Before he was shoehorned into the PPC, they were usually things such as "Thou shalt not go bats*** on someone who tries to intimidate you or boss you around and attempt to beat the living daylights out of them," so at least he's found a socially acceptable outlet for them now. He would probably fulfill many items on the checklist for psychopathy, save that cunning plans bore him and he's more likely to foil a Rube-Goldberg-esque plot to kill massive numbers of people than to come up with one due to his impatience with idiots.
Matsu began life as a character in a parody of "Neo Armageddon Evangelist," being a parody of the "Washington" character in that he bungled his versions of Washington's assignments spectacularly (Asuka woke up while he was groping her and chased after him in a rage, he was unable to kill Rei due to her reminding him of his mother, and while he was threatening Shinji and Misato, Asuka found him and he ended up having to scale the outer wall of the apartment building down to ground level to save his life). Fortunately for him, he was a rather amusing OC, and his author decided to keep him.
As far as backstory goes, he was trained as a super-soldier from a young age, and was SUPPOSED to be part of a "Defenders of Humanity" program. However, when he hit puberty, his inherited tendency to go into homicidal rages came to the fore, and one day, he decided to bust out of the government facility where he'd been raised and see the world. Several hours and many injured guards and wrecked pieces of government property later, he was out and on the streets. Fortunately for the inhabitants of the nearby cities, within a few weeks, Rei AR (Reiar) jumped in from an apocalyptic alternate universe and forcibly recruited him for an inter-dimensional protection force... one which she was SURE existed, but was quite fuzzy about its details. After a few months of camping in various dimensions, they stumbled upon the PPC, and she decided it fit. Matsu consented after asking that his inter-dimensional criminal record be wiped away, due to law enforcement officials being annoying.
For his and Reiar's initiation into the Department of Bad Slash, they were forced to watch a videotape of some snippets from "Little Miss Mary" and write a semi-complete charge sheet. Despite his attempt to strangle his partner afterward, they passed.