I believe it is the mc. The mc was weak back then, but the leader didn't want him to end up with his daughter, so he was sending him on suicidal missions against powerful enemies, and the mc manage to survive because of his tracking and assassin skills. Because of his tracking skills, he's also a master of getting rid of his tracks, so he's shadowless, since they couldn't find a trace of him or his martial art. Remember he was facing the divine blood palm tyrant dude, because the leader sent him on that final mission, in which would have killed him, if the artifact didn't take his side, and the leader was upset he didn't quit when he gave him the chance and was worried, his daughter would choose him. He just didn't know he had the nickname and also that's why he wrote a random name down, because he realized he's shadowless and is trying to confuse them.