Avatar (化神 Keshin ) is a family in Battle Spirits, shared by Spirits and Ultimates, which first appeared in BS44, the beginning of the Grand Advent era.
This family is not centered around any particular color, but instead correspond to individual Grandwalkers and exist in every color. All main set Grandwalkers have their corresponding Avatars. Avatar is in general the signature Spirit/Ultimate of the Grandwalkers, with traits that match the Grandwalker most in terms of name and mechanics. Game-wise, it is usually the center of the deck of the corresponding Grandwalkers, and most of the times act as the finisher or key card of the deck. Lore-wise, Avatar is usually the representative and projection of the Grandwalker, and one of the strongest beings the Grandwalker controls.
Most Avatar Spirits/Ultimates have Unleash or a variant of it, until Rebith era.