まるです。

一度捕まえた獲物（おもちゃ）は、なかなか放しませんよ。
Maru does not readily let go of the game which he caught.

そのまま箱に入ってみたり、
He is in the box with holding game in his mouth,

ひっくり返ってみたり。
and lies on his back.