Ok, Kain only turned twice. The second time made perfect sense if you think that he was only pretending to be good the whole time, and was just waiting to screw you over to get the crystals. Remember that there was no way Golbez could've got that last crystal without Kain.
For the first turn, anything could have happened since the the last time you saw him. It's clear even at the very beginning though that Kain is jealous of Cecil and doesn't want to break ties with Baron.
Also, Kain actually had personality unlike Celes and Cloud. And he was good in battle, while Celes was one of the most useless characters.