With vigilance keeping it ready both offensively and defensively, Agrus can turn a single-target boost into an entire team upgrade
Kokusho is the most famous of the five legendary Dragon Spirits, and its effect scales up well for multiplayer games, too, while the same can't always be said for its counterparts in other colors
Its only effect is to "celebrate" an opponent and then proceeds to list some examples, to represent the theme of appreciation for one's companionship that often permeates over the celebrations
Prowess is an ability that gives a creature +1/+1 until the end of the turn whenever you cast a noncreature spell, like an instant or sorcery, for example
In fact, in Legacy, I board it in against both Death and Taxes and Jeskai Control even though neither deck really interacts with their graveyard, simply for the previously mentioned reasons