Many characters help a ton. Kirby and King D3 are 2 of the big ones. Kirby was fighting from the start, saved both of the princesses at one point, rescues link and yoshi (or mario and pit), flies the dragoon into ganan and bowser's ship, destroying it, defeats petey, ect (you get the idea)
King Dedede put those badges on luigi and ness and kirby swallowed one. Without them, everyone would have been lost and defeated in subspace.
Sonic does help quite a bit, even though he has a very late entrance because he weakens tabuu significantly for the final battle.
And, this just IMO, but I think lucas helps too. I believe he defeats 3 or 4 bosses, more than any other character I think, and saves the PT from being destroyed after one of them.