... the other Sully brother! From AU! *coughs*yes, yes, I know, I'm an addict and here's another pup!*coughs*
In this Sully's universe there was no death the day before heading to Pandora, so instead of Jake, Grace Augustine had Tom Sully, PhD, in her team. Therefore Quaritch didn't approach him to undermine the program, and Tommy had his avatar, and spent quite a few months learning all about the ecosystems and more while there.
While looking identical, Tommy and Jake are definitely different. While Jake has all the 'street smarts' that enabled him to survive Venezuala, Tommy would have difficulty surviving the streets (well he didn't survive them, did he?!) without his brother when they were growing up, or Norm Spellman, or another buddy to watch out for him. He's easily sidetracked by biology, and all things that make it up, and while he had a PhD in xenobiology before he was 19, and has spent 3 years preparing for the Pandora project after being selected, he has zero skills when it comes to women, and very little experience! Drive an avatar, figure out the interconnectivity of a distant moon's fauna and flora and native species, piece of cake! Pick up and talk to a woman? Uhhhhh, Jake? Norm? Help?! Though he'd probably be able to tell her about her pheromones and cycle, and the part of it she is in after asking her rather personal and 'non-we've-just-met-5-minutes-ago' types of questions!
SO... in other words, heeeeeeere's Tommy!