Gon Freecss from Hunter x Hunter. This was the very first fully custom mod I've created, but even then it offers some unique (well at that time it was released it was nowadays other mods have it too) mechanic. Namely the neutral special can have 3 different variations depending on if the player presses up, down or doesn't press anything after the charge. This is true for the Final Smash too, but in that case it's only two variations.
Put the data file into your SSF2's data folder and don't forget to make a backup.