1. You're supposed to bet the Striker, not the Assassin.
2. Did you wait for Shadow on the Floating Continent? If not, you're out of luck.
3. If you did wait, have you gone to the Cave on the Veldt yet? This is where you'll find Shadow (if Relm is there, then you didnt wait on the FC). You'll go back to Thamasa with Shadow. Leave, then return, you'll find out he's left. Then go to the Colliseum and bet the Striker.
4. It's Chupon.