Krillin

Arriving at the Turtle School as Goku’s rival, Krillin quickly becomes his training partner and then his most constant friend. His narrative function is valuable: he provides the scale. When Krillin can no longer keep up, the opponent has changed category.

He is also the one who makes the decisions the heroes cannot own, and the one who dies whenever the series needs a trigger. Unlike many supporting characters, he accepts his ceiling clear-headedly and keeps showing up.