Air Wall's name is Conjectural. No official name has been given, and it is subject to change at any time.
Air Wall lets the user create circular shields made of compressed air from their hands in order to shield themselves. These barriers are strong enough to withstand powerful attacks such as Izuku's Delaware Smash Air Force. The barriers can be layered for greater protection, and the user also has the ability to create a dome barrier around themselves to guard against attacks coming from different areas. When under the effects of a Quirk-empowering drug, Nine was able to create a large, rectangular barrier.
Nine can also use the shield to blast people away or fire powerful bursts of pressurized air able to knock down and even stun the opponent.
Although extremely tough, the Air Walls are not indestructible. They seem to have more durability the more Nine is focused on them or is in good health. This is shown when Nine was physically weak due to his condition and the Air Walls just shattered upon impact compared to when he's at full strength and they are highly durable.
Air Walls are only created in the direction Nine raises his hand, so agile enemies can maneuver to avoid the shield and attack unprotected areas.