Confirmed AT's

Assist Trophies are basically pokemon, though they are characters instead.

-Duster
-Rosalina/Rosetta - throws out star pieces for a few seconds, and that if you pick them up, you'll recover health. A Luma comes out with her.
-Sothe from Fire Emblem Series - because knives are banned, he attacks with a sumersalt kick instead
-Lakitu
-Group of Pegasus Knights from Fire Emblem Series - "they attack in a group or something, with a Falcon Knight"
-Jill Dozer ("drill girl with pink hair" but no doubt there)
-Birdo — Shoots eggs at an opponent. Pretty standard Assist Trophy. Oh, and he's fully rendered; it's not a 2D sprite like the Melee stage hazard. (needs confirmation)
-Tingle — Like the Nintendog or Mr. Resetti, Tingle doesn't inflinct any actual damage; he just stands there and does a little dance while shouting "Tingle-Tingle, Kooloo-Limpah," then disappears. I like to think of him as the Goldeen of the Assist Trophies. (needs confirmation)
-Raiden — I'm not a big fan of the Metal Gear Solid series, but I think this is Raiden (he's the guy with the white hair, right?) He just attacks with a sword before disappearing. Didn't seem very interesting to me. (needs confirmation)
-Samurai Goroh — Slashes Away at opponents
-Dr. Wright — Summons Buildings
-Knuckle Joe — Appears and does the Vulcan Jab, Smash Punch, and Rising Break
-Nintendog — Appears and Paws, doing damage
-Devil — He uses his powers to move and attack the opponents
-Andross — Comes in the background and spits out polygonal tiles which deal damage
-Lyn — Disappears and then appears slashing the opponent dealing lots of damage
-Mr. Resetti — Very annoying and starts talking in a middle of a stage saying lectures and other crap
-Excitebike — Excitebikers come and attempt to run you over.
-Little Mac — Appears and dash punches, jabs, and then uppercuts
-Gray Fox — Hits opponents with his sword
-Saki Amamiya — Cuts and then shoots
-Stafy — Explosion and Spin Attack
-Waluigi — Tennis Racket to hit, stomps out characters, and kicks them