As with most homunculi, Paddy is a copy of the human body, right down to his genitalia. His flesh even feels like human flesh. In terms of appearance, his skin is an odd shade of brown, with no hair on his head or body. This is partially due to regulations enacted by the White Council, which places limitations on homunculi - especially homunculi that look exactly like a human.
His responses to spells similar to that of an actual human. If he is injured too badly, he goes into regeneration mode and rebuilds himself.
Lady Barb warns the students that Paddy is to be treated with the same dignity as a human being. Maliciousness against him will be treated with the same severity as maliciousness to a living person.