Some manner of larva. He is a creature whose body is given substance through the power of a high elf.

Larvae are beings of spirit from a different plane of existence, but in some cases the powerful magic of the high elves can allow them to manifest on the material plane. Abilities and appearance vary from creature to creature, usually determined by the situation in which they are summoned and the thoughts of the high elf who summons them. Borgnine himself says very little, and follows Ellis more closely than her own shadow.

He will risk his life to help Ellis, but will not help anyone else without her order.