Sister Mary Sue goes out several times a week for cocktails dressed in something that's Stripperific or even Vapor Wear. She cusses so much sailors have been known to faint from shock, and is a regular at the local No Tell Motel. With FatherMartin, no less!
When no one's around, Sister Mary Sue likes to have a cigarette and a stiff drink and give herself fancy hairdos.
Sister Maria is pious but a bit too whimsical and immature to be a nun. She enjoys singing songs, climbing trees, and sneaking off to go on walkabouts in the countryside, and is often late for prayers.
Sister Mary Sue is in a Nuns Are Mikos or Naughty Nuns setting, where her behavior and that of an actual nun may be two very different things.
Sister Mary Sue is very religious, but also has little to no self-control. Thus making her do things she will regret.
Sister Mary Sue is a decent person who sincerely does her best to live up to the ideals of her Noble Profession.
She likes to go out and do fun things society does not associate with nuns, but doesn't do anything bad or unwholesome.
Double Subverted: ...In front of her Mother Superior. When Mother Superior isn't looking, Sister Mary Sue kicks puppies and jaywalks on her way to the No Tell Motel to meet Father Martin, with cocktail in hand.
Invoked: Sister Mary Sue acts like this so she'll get kicked out of the convent she has no desire to be part of in the first place. (Or no longer desires to be part of.)
Defied: Sister Mary Sue strives to be the perfect nun.
Discussed: "Um, Sister Mary Sue is a little...different. We really wonder why she's here at the convent of Our Lady of Tropes in the first place."
Conversed: "They actually let her join their convent?! The Catholic Church must be really desperate..."
Deconstructed: Sister Mary Sue is an outsider in the convent, the other nuns blatantly disapproving of her Nun Too Holy behavior. She isn't a bad person, she just couldn't give up some of her vices. She may not even want to be in the convent, but was sent there by parents who wanted to get rid of her.
Sister Mary Sue eventually gives up her vices, or at least learns to control them. Eventually the other nuns start to trust and accept her. Soon enough, she becomes what one would expect a nun to be like.
Sister Mary Sue channels her urges to have fun in a more acceptable direction. She takes her fellow nuns to the amusement park, goes rollerblading, starts snowball fights, and engages in various other pastimes that may seem un-nun-like to outsiders but don't go against her vows. She livens up the convent and the other nuns become fond of her.