+SPD/-HP or +DEF/-HP Winter Robin

+Spd. If you refine his Tannenboom for speed, he has 40 on offense and 42 when being attacked. Assuming nothing else is affecting it. That's while still commanding a very nice 35 def.

Depends on what you look for in a unit; offenses or defenses. +Spd makes it so that you double a lot of stuff you couldn't before with Brazen Atk/Spd. +Def makes it so that can run Ignis sets more effectively & take minimal damage from melee units. He can work in both phases, but if you want a more Player Phase-oriented build; +Spd, & vice-versa for +Def.

If you wanna merge him; that's fine, too. But I'd say keep him if you're looking for Tannenboom!+, Brazen Atk/Spd, or Armor March fodder.