"The knight's ability should have the flavor of protecting others, with a gameplay advantage for the player."
But, as pointed out, in a competitive game you may not want to give the other player such an advantage. To that end, this approach would seem best for those scenarios where you *must* have the help of other players (and thus welcome the boon granted by a knight, and not mind as much the added benefit given back to him). Take, for example, a more epic boss-level monster, that requires team effort to defeat. I can easily see the knight being a great character in such a game. And if you can devise scenarios where team-work is needed often, maybe the knight could get a personal advantage not every time, but maybe for every 3rd damsel (i.e. other player) saved/assisted, to even it out a bit.