Jarin's Level Up Ability is best activated as soon as possible, as it gives the player a much needed Soul, either to stay ahead of, or to catch up with, an opponent.
Once the Level-Up is triggered, it adds one Soul to your total for the rest of the game. Players cannot remove this soul in any way.
On Jarin's robe, there is a picture of a skull which resembles Xyzax's head, another Boss in the base set.
Jarin also seems to appear in the new All Your Base Spell card, which features him without the skull headpiece, and appearing similarly to CATS, a villain from the game Zero Wing.