I think that your characters are good, but I think if we made them a little bit "mashed together," we could make a great cast!
Ky: Liam Hemsworth (just because that is how I saw Ky, not even thinking of that actor at the time.)
Cassia: Bella Thorne. she is the most outgoing person I know, and I will mention the role to her on Wednesday when I see her. (she's my bffs cousin, so we're pretty tight. ☺😉.)
Xander: Hunter Parrish (don't know why I didn't think of him, perfect!