I'm not very knowledgable on characters. Basically, for this, you need 2 characters from a series; 1 that's very shy and quiet and any other.
The story is, they go on a camping trip and the shy one soon needs to "go". However, the toilet facilities is just an outhouse with 2 holes next to each other, no partitions. The girl tries to hold it, but after 3 of the 5 days they are there she can't take it anymore and sneaks out at night to try and empty herself in privacy, but as she gets seated someone walks in.
Follow that guideline and you can't go wrong for this challenge. However, the girl must not be won over; she is as humiliated upon getting relief as she is for having to go in public to begin with. Just some extra notes: her crap will obviously hurt, since she's been holding it 3 days, but no exaggerated grunts because with her being so embarrassed about it she would be as quiet as she could. Also, you don't need to describe the preceeding days in detail, just mention that she's been holding it.
Oh, and yes, the other girl who comes also takes a crap.