Just an average airbender who serves as an average avatar.
I like to think that during his entire journey, the fire nation are looking for the avatar but keep overlooking Saitama ‘cause he doesn’t look that strong and assumes he dresses the way he does because he must be an Avatar fanatic or something (”must be from Kyoshi island”) When the world is saved in the end, nobody knows who did it and assume it was the combined efforts of the Kyoshi warriors and the Dai Li or something.