Truman was kicked out of his old school for stealing a car from one of the teachers. So he came to Casey's school where he instantly fell in love with her. Truman is like Derek, he comes up with plans to get a girl. Like getting her jealous.
Truman rates all the girls in his grade out of ten and gives Casey a 6.5. Not because she wasn't pretty though. He did it so she would start noticing him Which is when Casey then started having dreams about him and develops feelings for him. Soon after they start dating. But they break up in the episode "Truman's Last chance" and get back again in "Surprise". However, they broke up yet again before vacation with Derek.