Guys, who said anything about 'Marth' being an antagonist? Maybe IS is taking inspiration from other Nintendo Media and 'Marth' is akin to anime!Meta Knight in Kirby of the Stars. Of course, that's not exactly confirmed, but I don't get an antagonist vibe if 'Marth' is seen helping out Krom kicking Corpse Soldier ass.
From their reactions in the games, here's what I think happens in the game-
1- 'Marth' challenges Krom to a duel in that coliseum place to test his mettle.
2- 'Marth' helps Krom and Liz against the Corpse Soldiers.
3-'Marth' tests Krom's mettle again somewhere else.
4- Something happens with 'Marth' and that eye thing.
Of course, because there's not a lot of cutscenes, I can't really tell what's really going on, but that's my interpretation of all the 'Marth' cutscenes. So yeah, I don't get that antagonist vibe... I feel more like 'Marth' is being a teacher figure to Krom.
And I still firmly believe that the butterfly motif isn't a coincidence. *shakes fist*