Companion - One who is there for the entire journey and aids you
Ally - One who joins you at various points and aids you occaisonally
Companion - Navi, Fi, Ciela, Tatl, Midna, Epona
Ally - Raru, Impa, Zelda, Llia, Linebeck
Ezlo, he's there with you every step of the way and doesn't shy away from helping link anyway he can. Plus y'know he's actually useful and an integral part of the game and not there just for plot points.