As in, can the engine handle it? Does the current code allow it? Luigi would be relatively simple sprite wise for the most part, just recolor Mario's animations, i'll make Poltergust attack custom sprites if i find it out to be possible to add characters though.
His niche could be inflicting powerful status ailments on enemies, maybe a few buffs too, with above average defense, magic and magic defense but meh offensive stats to balance it out, his ultimate special could be Negative Zone, the animation just a big circle that inverts the colors inside, green fireballs, lightning and such would all be simple spritewise too.