Kaede works like this:
If you convinced her after the party to stay, she will be there that same night after the party, then she will also appear at night only when it rains - if you buy and use "rain stick" in the morning, it will rain that night, thus spawning in kaede in Yuna's room
As for interactions - there are 3 with her:
- blowjob
- prone bone (unzip sleeping bag, must be laying on her belly)
- missionary (unzip sleeping bag, must be laying on her back)
I don't remember exactly how, but you can turn her around if the sleeping bag is zipped I think
She's just a side kick for MC to have some relief