Resistance Pilot is great in my opinion. She (I'm sure it's a she based on the death scream) is a staple on my arena team.
Although if you don't really intend to use any of the other characters you could always just unlock the characters at their lowest star level (for the unlock X number of Dark/Light side characters acheivement) to ulitmatly get a hold of Vader shards.
Or even just to try them out on the lower difficulty battles to see if you like them.