I get the sense we're a bit frightened of going too deep for fear of hurting someone's feelings... so I'll start with Shadow.
I agree that he's upper class and probably at university. Probably flunking out and without close friends, Getbig became his outlet for awhile. Will finish school eventually and will put his English skills and accumulated knowledge of "the west" to good use at work somehow.
Donkey Kong - One of the few nice Germans.
Probably a very gentle guy in real life, all about peaceful resolution. You wouldn't want to piss him off. May have a collection of sheiss movies stashed in the attic. j/k
Possibly in a relationship - she's not fat. All around optimistic guy.
Goudy - Working class Irish family, lives at home, probably no uni. May have been slightly geeky in younger years and has developed some "loner" tendancies which pushed him away from team sports and toward weights. No lasting relationship since Flossy, and he's probably starting to worry if he'll ever find someone he can love again. Wishes mum would quit pushing him to find a nice girl. Determined as hell to turn lemons into lemonaide, and probably will.
TA - Attended uni but quit because he couldn't stand being a social outcast just like in high school. Angry dude but probably very quiet in real life. Started lifting with an "I'll show you all!" attitude, which he still maintains. Getting older now without a career and is starting to really worry about the future. The "self-loathing Jew" thing reeks of father issues and is probably the real key to his psyche. Owns a handgun, probably a Lugar.