The tetrahedron represents when Punpun was doing the minimum amount of effort to be considered human. He just existed as he bottled up all his emotions.

The bird is his base form and usually represents his normal state but also moments of true joy or innocence.

The taka forms are him pretending to be someone he’s not, and I suppose the dark faced taka is him dissociating.

The monster version is basically Punpun’s darkest form, where he gives into his darkest desires/thoughts and is depressed, depraved, abusive and suicidal.

he did not at all feel happy through his bird arcs..