Someone who has no siblings. Some upsides are getting what you want, having some friends that are only children, no being irked, and having higher goals. Downsides are lashing anger on kids who have siblings, and can end up being a jerk. Sometimes it leads to having an inflated ego.
Friend 1: How's your sister?
Friend 2: Pretty good. She's playing soccer.
Only child: I'm frickin' tired about my brother that, and my sister this! Screw all of you! *walks away*
Friend 1: God, what a jerk.
Friend 2: I know.