Arborus is a 6 star First One. Its element is Earth.
First Ones can be summoned during battle by tapping on your character at the bottom right of the screen once your energy bar has been filled. Energy is generated every time your Arkadions deal and receive damage. First Ones will fight alongside you, but cannot be controlled. The energy bar is reset to zero when you summon a First One or enter a town. Each First One can only be used one time until you return to a town.
Obtained through story quests--after defeating him in Arborun.
First Ones gain strength every time they are used.