In Chapter 3 of "A Separate Peace," state two of Finny's major character traits.

In Chapter 1 Gene explains one character trait of Phineas: 

Phineas...considered authority the necessary evil against which happiness was achieved by reaction, the backboard which returned all the insults he threw at it...[he]combined a calm ignorance of the rules with a winning urge to be good,,,and never more than when he was breaking the regulations..and in the truant's corner.

Somehow Finny is able to disobey without angering the faculty too much.  The faculty "threw up its hands over Phineas" and loosen the grip on all the boys.

There is a carelessness (insouciance) to Finny.  He forms the "Super Suicide Society of the Summer Session," tempting fate as the boys jump off tree limbs into the water; persuasive and influential, he is able to induce Gene to do things that he does not want to do, such as jumping.  Gene reflects, "Every time, when I got myself into position to jump, I felt a flash of disbelief that I was doing anything so perilous."

Phineas perceives sports as the "absolute good."  Phineas declares that when "you play a game you win," never permitting himself to realize that when he and his team win, the others lose.  He creates the game of "blitzball" after Germany's blitzkrieg, the tremendously fast-moving army crushing whomever was in their way.  In blitzball everyone is the enemy to be knocked down.  This game "brought his own athletic gifts to their highest pitch," Gene comments. "Finny sould shine with everyone, he attracted everyone he met." His confidence, athleticism, and leadership in sports have powerful influence on others.