Fighter 84: Chibi-Robo

Chibi-Robo is a small robot from the Chibi-Robo! Franchise. He is a first-party character from a series that was not added in Ultimate, so they were an obvious pick.

Trailer
The trailer starts with ROB and Mr. Game and Watch battling. ROB is winning by a lot, but suddenly short circuits in the middle of the stage. There is a glimpse of light out of ROB's neck, followed by sparks. ROB starts working again, and the thing is flung off. It zooms in on the small figure, and it's Chibi-Robo, followed by the message-

CHIBI-ROBO CHARGES UP!

It shows a bunch of Robo's moves with the song Chibi-Robo! in the background. It shows the final smash, and cuts to one of Olimar's white Pikmen with Chibi-Robo on the PictoChat 2 stage.

Statistics
Chibi-Robo is one of the lightest characters in the entire game so he is by Jigglypuff in terms of weight.