Mario

He got stuck in a painting again, didn't he. So now Luigi is out to save him, although last time, wasn't he in a washing machine?

In this game he does nothing
Mario has yet again been captured by king boo and put in a dark light activated painting. Mario is really only seen once to a very few times in the whole game, and must be saved by luigi. Mario does not do anything has he has been captured, and is not even seen in scarescraper.

The end
Mario is at the final cutscene where the dark moon is pieced back together. He is freed from one of the dark light paintigs.

The only reason King Boo kidnapped mario is probably to lure Luigi in, but the little bro actually saved mario, AGAIN.