Wheatley

Wheatley died in his early teens, upon his death and transformation into the undead, he gained the skeleton markings all over his body. he still grows up the same as if he were living, but he is immortal.

he has major depressive disorder, it's not uncommon for him to have angry outbursts because of this. 

Wheatley hibernates most of the year, he wakes up roughly from September to November. Sometimes staying awake longer or shorter.