Sister of Mercy is a Sheo-model, a generalized character of a girl who is so good that there is no need marrying her. To be more precise she is not that good. No. She is actually good neither as a mistress, nor as a wife.
She is no good not because she is bad but because that’s what she thinks of herself. She probably waits for a boy to offer her everything. Or she may be shy thinking she has no right to demand. Or she may feel that she does not deeply excite him yet (as she hasn’t learnt the first four lessons of Columbine sister, for example).
Sister of Mercy is a girl that would have been a perfect wife, but the guy she is dating, already has a wife. And he is not willing to change her as to make a move like this he needs this fire that Sister of Mercy doesn’t have, or better say, she does, but she hides it and chooses to agree with anything and help her man out with everything she has. Sister of Mercy is very comfortable for no strings attached relationship.