Solid Air allows the user to solidify air to form a wall or platform through their breath.
Air walls do not fall to the ground once solidified and will stay afloat in midair even when another person hangs onto it. They're soundproof as well, making it effective against Quirks that rely on sound like Anivoice or Brainwashing.
The size of the air barrier depends on Kosei's lung capacity. The barriers are breakable, with Katsuki Bakugo being able to punch through one of them. By the time of the Joint Training Battle, Kosei has improved his Quirk, making his barriers much more resilient, so much so that Tsuyu Asui and Denki Kaminari needed metal pipes to break them.
Kosei uses Solid Air to create nigh-invisible transparent barriers for defense, preventing opponents from getting close.
After strengthening his Quirk, Kosei became able to create more elaborate and durable air constructs, like an air cube that can restrain people inside it while cutting off their voice from leaving to the outside world. He does this by shaping his hands around his blown air.