Dr. Genus experimented on Zombieman as part of the immortal series. He escaped their clutches and ever since hunted for any clues on the House of Evolution's whereabouts. After confronting him, Zombieman realized Dr. Genus' had truly given up on finding the perfect creature and decided to let them go.
Zombieman dislikes Sweet Mask and was not willing to be put under his control. He would give up his credit for the Monster Association raid if it would convince Sweet Mask to lead the surface strike force.
When Saitama heard of Zombieman's immortality by Genos he became interested in the S-class hero and asked him if the rumors were actually true, although Zombieman looked rather displeased by having Saitama around, due to his rank. When Dr. Genus later warned him about Saitama's strength, Zombieman refused to believe him, even thinking that the former scientist was making fun of him.
After witnessing Saitama's fight against Garou however, he completely changed his mind about Saitama, realizing that everything Genus said about him was true. He then visited Genus again and asked him to remove his limiter as well and to make him equal to Saitama, as he started to feel inferior to the B-Class Hero. Zombieman was shocked, when Genus told him that he can't do so, but the scientist still decided to help him, by showing him something "interesting" in his basement.