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.