Assist Trophy: Eliwood

 Eliwood would make a great assist trophy. As Roy's father and protagonist of the first American Fire Emblem game, he definitely qualifies too.

Assist Trophy: Miciah

Very deserving of being a character, but there's simply characters who deserve the slot more than her. I can see her very likely appearing as an assist trophy because of her importance to the series. Maybe she could call her bird to attack you, or use magic.