Johanna Martez - It’s more of a backstory for the time being. I’ve seen that they’ve been prefects so they probably knew each other back at school.
Phaere White – I’m not sure if there’s exactly a purpose if you thread with Kastor. I just see a friendship between them.Perhaps talking to each other from time to time. Kastor visited the forbidden forest once to draw something and I’m certain that he would do it again. If Phaere would be up to break the no visiting the forest rule, they could accidentally meet there and get into trouble by encountering creatures.It's up to you how their thread will roll.