So Elphaba probably takes a liking to Terezi immediately as a fellow member of team "okay but can we not murder people for no fucking reason" and also, if she's forced to admit it, as someone who looks even less like a human or munchkin is Supposed To than she does (though thankfully she has the tact to not put it that way). Up until now her other friends have all been social butterflies who just aggressively befriended her so chances are it took a little longer for her to start treating Terezi as anything more than a casual acquaintance, but they're pretty alike so I imagine that once that happened they started getting along pretty well!

Also I mean, nothing brings people together like a visceral terror of water right