lol...you need to feed them gems or koma something like that....
go to the main menu,
if you have the translated one,the menu has round "buttons'right
go to the 5th one...read its description--it will say---this one will power up your characters,go there...then
just fiddle around..for example go to naruto--push x or y --if you stumble upon a "FAMILY BRANCH THING"..thats it..use your gems to power them up..so you'll get newer characters..
ps---im not really sure,i dont my ds with me this moment...