Clover

Clover is the protagonist of the Undertale fangame Undertale Yellow, a child wielding a gun and wearing a cowboy hat. Clover has the yellow SOUL of Justice. The game doesn't specify Clover's gender.

They are the 7th human to fall down into the Underground, and they did so intentionally by jumping down the hole at Mt. Ebott in order to look for the previous humans who also fell down and went missing.

Martlet: Clover, are you sure about this?
Ceroba: They seem serious...
Martlet: Clover... You are the kindest, most selfless souls I've ever met!