Finny is charismatic. There are very few people in life who possess this quality. They have the ability to get anyone to listen to them (Finny does this not only with the other boys at Devon but also with his teachers). Charismatic individuals are also popular without trying to be. Even Gene, who is a loner, is drawn to Finny, and Leper--the eccentric--is fascinated with Finny. Because of Finny's charisma, he is able to convince others to do things that they normally wouldn't think of such as Gene's jumping from the tree or training for the Olympics.
I would call Finny competitive.
Finny creates games and circumstances so that he can complete with the other boys and come out on top. Blitzball is an example of this as well as the snowball fight. When the boys have the Winter Carnival, even though Finny isn't quite able to participate, he works to make the attractions of the carnival the best ever as if he was competing against some past event! Even in his social circumstances, it seems he is striving to be the best. In chapter 2, it seems that at the Headmaster's tea, Finny is looking to be able to impress or speak and entertain more than any of the other boys are able to.
The word that I would use to describe Finny would be thrill-seeker.
Throughout the novel we see Finny's larger-than-life character extending himself to be just that, continually challenging himself to come up with something better and more risky to feed his needs. We see this in two specific acts during his Sarcastic Summer. First, he creates the "Super Suicide Society of the Summer Session" with its initiation rite of jumping into the river from the tree--foreshadowing the tragedy to come. Second is the game of Blitzball where no one wins--another bit of foreshadowing to the overall tone and theme of the novel itself.