CW: Body Horror and other such things that were warned on the front page! They may all exist here.
Insides
Canonically speaking, the invasive part of Insides is described triggering the β-Endorphins in the brain. The author describes it in paraphrase as frightening and enjoyable because of the endorphin release. Hikaru, however, finds himself disgusting when he leaks out his true self and at times finds it embarrassing when people see that side of him. He only does it when he is hunting someone, is truly stressed out and doesn't know how to handle the emotions that humans go through, or if the need to act and use that form arises.
Below is visual representations of Insides and their infiltration. Be aware that the warning is there for a reason if you click the dropdown.