The Oak

From Inga no Kusari wiki

The Oak is Finn Green's Glint.

The Oak is a small bronze pin badge in the shape of a large oak tree, it measures about one and a half inches tall. And on Finn's jacket, it's positioned beside the Staff badge.

Glint Ability

The oak is a symbol of strength and endurance to many cultures. The Oak badge that Finn wears grants him an extra 0.5 times reiatsu capacity for each Fullbring level he possesses (at a single Level 5 state, he'd have an extra 2.5 times the space).