Youtube user RED has modded Super Smash Bros. Brawl to make Chrom from the Wii U release a playable character. RED was able to modify the character files and replace Marth with Chrom.

This is just a model swap really, but regardless, it's fun seeing non playable characters fighting in general. You can check out the video below showing Chrom vs. Kirby.

Update: the original article erroneously stated that the game involved was Super Smash Bros. for Wii U. The article has been update to reflect the correct information.