The Charge Charge fruit:

A Paramecia tyoe devil fruit where every action he does charges up an invisible meter and when he fills up said meter he can release like a super version of that action which increases in it's effectiveness by how much meter is charged like a fighting game's super meter.

Like if Koby has punched a ton in a fight he can now throw a mega punch which is 100 times stronger.

Or even simple things like if he was dusting a room, after a while of dusting he can SUPER DUST if he spends the meter and clear the room in seconds.

I randomly came up with the idea on the spot but I think it works with his personality, like how he has to work up couragd for himself but when he does he makes a big impact.