Spoilers for One-Punch Man chapter 169 (JP) ahead!
With the One-Punch Man manga's most anticipated fight between Garou and Saitama now behind it, it turns out that there's only one character who actually remembers what happened--and it isn't either of the combatants.
The battle between Garou and Saitama was initially fairly one-sided since Garou desperately wanted to fight and Saitama wasn't all that interested. Garou figured out one way to finally force Saitama to get serious, however, and that was by killing Genos and ripping out his core, which seems to function as both brain and heart to his cyborg body. Saitama and Garou's battle escalated rapidly from there, with portals, deadly radiation, the warping of spacetime, and Saitama destroying Jupiter and Io with a sneeze. The fight ended when Saitama actually transcended causality, throwing a punch that landed hours before it was thrown and soundly defeating Garou before any of that could happen.
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This briefly led to there being two Saitama in chapter 169 of One-Punch Man--the one from the future who landed the punch, and the one who existed at that time and was still lost amongst the rubble of Monster Association HQ. They were quickly integrated back into one being, a Saitama with no memory of that battle in deep space but who was still holding Genos' core all the same. Since this took Saitama back to before Genos was killed, he found himself looking at the badly injured but still alive Genos before Garou killed him, little more than a torso and a head. Intrigued by the duplicate core, Genos connects with it, and immediately witnesses everything that happened in the alternate timeline, including
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