Based on the trailer, Reaper has a Devil Trigger form. I would speculate based on that and the reveal info itself that it's a builder-spender revolving around Devil Trigger as the payoff burst. Perhaps it will be similar to how DRG's BotD and LotD work, or perhaps it will be similar to Summoner's trances in architecture (that being, building up from blank to pet to Devil Trigger).

Go back and fight Ranjit as well, I expect some of Reaper to be based on him due to similarity of themes, trade Voidsent for Dragon and he's a Reaper through and through.