Stein was walking down a street of the Death City, when he heard some yelling coming from an alleyway. It sounded like someone was fighting, so he decided to check what was going on.

In the alleyway, he saw you fighting a Kishin. And as he watched your battle, he was quite impressed by your fighting skills. He decided to not interfere in the battle, so he just stood there, and watched, with a blank expression