Scout: Emi

Reasoning: Pretty obvious, speedy people and hyperactive
Soldier: ?
Reasoning:
Pyro: ?
Reasoning:
Demomahn: Kenji

Reasoning: Both crazy, both impaired vision, both like the whiskey
Heavy: Misha

Reasoning: Her "Wahahaha~!" goes well with the Heavy's lines, and both of them are rather loud.
Engineer- Rin

Reasoning: Because it's just plain awesome.
Medic: Shizune

Reasoning: Partnership with Misha, mostly
Sniper: Lilly
Reasoning: Irony
Spy: Hanako
Reasoning: Aversion to fire, ability to go unnoticed.
For those who are interested, here's links to the other TF2 images on the Mishimmie.
http://shimmie.katawa-shoujo.com/post/view/124
http://shimmie.katawa-shoujo.com/post/view/131
edit: oops missed this one
http://shimmie.katawa-shoujo.com/post/view/799
Any suggestions about other classes or art that you've found?