Your route is determined by the first 6 choices. Each character prefers certain answers and whoever you choose the most answers for is whose route you start. If you manage to balance the answers you get to choose your route yourself. Additionally, Xanthe requires having completed the good and bad ends of the inital 3 unlocked routes, Lien requires completing Xanthe's, and the secret route requires completing Lien's.
I can give you the answers for each route if you need help with starting any of them.