Andross
Andross, from the original Star Fox, will appear, move back behind the action, and begin firing polygonal panels at the combatants.
Devil
From the game Devil World of NES (never released in USA) comes the Devil. He will choose a direction and point, forcing the screen to scroll in the same direction.
Dr. Wright
This character from the SNES version of SimCity blows away your enemies with skyscrapers that appear from the ground.
Hammer Bro
The Hammer Bro, originally from Super Mario Bros. will chuck hammers at opposing players.
Knuckle Joe
Straight out of the Kirby universe, Knuckle Joe comes out with a devastating Vulcan Jab and will end with either a Smash Punch which shoots straight through the enemies or a Rising Break which is a super powerful punch straight down.
Lyn
Lyndis from Fire Emblem crouches and waits, then vanishes before unleashing a powerful strike at a foe.
Nintendog
First appearing in the E3 2006 video (long before Assist Trophies are actually revealed), a Labrador puppy from Nintendogs jumps up and blocks the screen.
Samurai Goroh
Samurai Goroh, from F-Zero, will slash his sword wildly while chasing after opponents.