Alice is a villain who is completely insane but who operates more or less secretly, and would likely be locked up in an asylum if she showed her real nature, so he pretends to be happy and polite.
Alice is a hero who is completely sad and depressed, with little or negative self-esteem, but to help her friends and everyone he cares for, or to hide her insecurities, she pretends to be happy and cheerful.
Alice is in denial about something that deeply disturbs her, and constantly tries to ignore it.
The character seems to smile a lot and has been hiding some amount of depression, but his mask cracks and slips so often one must begin to wonder what is really happening inside the character's head.
Alice is a Mood-Swinger who always smiles, but sometimes it's real, sometimes it's not.
Averted: Cheerful characters are cheerful, and aren't hiding any dark inner feelings at all. Depressed, hollow, and insane characters look and act depressed, hollow, or insane.
Enforced: The series has taken a turn for the Darker and Edgier, and now Alice looks really out of place, but putting them on a bus wouldn't work... I guess we'll have to give them some secret angst or madness to let them fit in.
"I alway knew Alice was hiding something! That perpetual cheerfullness isn't a natural thing!"