Knight: Someone who serves and protects their aspect, and serves and protects others and themselves through their aspect. This is accomplished by either using it as a weapon or making more of it, as there usually not very much of their aspect in their party. 

Breath: Wind, freedom, particularly narrative freedom. Not being tied down. Going along with the flow of events, not being an obstacle to a story

A Knight of Breath would simply be someone who fights to preserve and protect their sessions freedom. If there are any obstacles holding their teammates back from accomplishing their goals, The Knight will be able to utilize wind in their combat to figure out a solution. They would be someone who's extremely difficult to capture or hold down, as most breath players probably would be.

They would be an extremely important player, as a Knights aspect is not only usually lacking in it, but the lack of that aspect is what defines the sessions greatest difficulties (beta kids had a reckoning that starts super early, beta trolls were in opposing teams and the teams themselves had a bunch of infighting, and many alpha trolls were out of their fucking minds). A lack of Breath would mean that there's some kind of oppressive or even narrative force that holds back their teammates from progressing. This could mean many things, but particularly severe examples would be if they were stuck in a null session, or had a carapacian that screwed up the way the game mechanics in Skaia, Prospit, and Derse work. Doesn't have to be a Jack Noir, if Warweary Villein (aka The Mayor) had actually succeeded with his uprising that got int interrupted by Jack, that still would have messed with the beta kids game.

They would be much more combat focused than an Heir, and could probably deal more damaging attacks in exchange for having to constantly fight for their freedom, rather than it being something that comes naturally to them. The wind will never save a Knight of Breath, it must be something that they learn how to use.

An extremely powerful Knight of Breath wouldn't gain new abilities, but the abilities they already had would be an extremely terrifying force to be reckoned with. Maybe they could even fight the author of the comic? This was meant to be a joke but I guess characters in Homestuck do do that, but a Knight of Breath would only fight the author if the author was intentionally screwing their characters over and trying to give them a bad ending, I guess.