affable personality
A personality that is friendly, approachable, sociable, amicable, and/or likeable. He might not be the best instructor in the school, but he has such an affable personality that he is everyone's favorite.
Jekyll and Hyde
A person whose personality drastically changes between good and bad. A reference to the main character in the book Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson. Ever since Kyle became a teenager, he's turned into a real Jekyll and Hyde, and it's impossible to anticipate his mood at any given time.