One of the starting champions for new players, Cassie is actually my personal favorite of the whole game's roster.
She doesn't have a fancy move-set or anything. Compared to other weapons, a crossbow would seem weak, but Cassie's arrows are powerful. I like her dodge-roll move as well, as if you're in a battle where you need to run before you get slaughtered, the roll can save you more times than you'd imagine.
Her special super-move is her eagle can scout the location of all enemies where you and your teammates can see them through walls. It doesn't last too long, and it's not that great of a power.
With powerful arrows that work even well at long range, I did really well with Cassie. Most games I play as her, I usually win, unless my teammates are morons. I oddly don't see many other players pick her, their loss I suppose.