mother
Somebody who can be a really nice person, but can also physically and verbally abuse you sometimes. Perhaps she does it out of anger, or hate, or whatever the reason. This is an actual conversation I had with my mother:
Her: Did you do something stupid?
Me: What? No! Why did you thnk that.
Her: Because ou're you, and you do stupid things (Ouch!)
Me: Hey...shut up!
Her: What did you say?
Me: Just stop! I don't need to listen to this!
Her: (smacks me across head) You don't tell me to shut up. I tell you to shut up. You can go to your room after dinner.

Me: (sigh) I wish my mother could just be grateful I exist...