1. Inherently; essentially. In this usage, "the" is used between "under" and "skin." She's really a sweet girl under the skin—her sassy attitude just gets in the way of that sometimes.

2. Said of something that is a source of irritation. In this usage, a personal pronoun is used between "under" and "skin." I just can't stand Dean's voice, so everything he says gets under my skin.

3. Said of someone or something that one is obsessing about. In this usage, a personal pronoun is used between "under" and "skin." I can't get that beautiful girl I met earlier off my mind. She's just under my skin now.