Genos' Core

Genos' body is powered by an energy sphere labeled Core (コア, Koa). It is attached to his body by a single cable and is held inside his chest. This is Genos' main source of energy, effectively his heart. Destroying the core or removing it from Genos will result in the cyborg's death.

Near the end of his fight against Mosquito Girl, in an attempt to self-destruct to take her out with him, Genos's core was seen filling up with energy and ready to explode. During his attempt to destroy the meteor, Genos removed the core from his chest and placed it in a similar holding place in his arm, to direct all the energy into his Incineration Cannon, making it more powerful. During his fight with Psykos and Orochi, it transpires that the core is a nuclear reactor.

In addition, the core can record everything happening around it even if Genos is dead. It could transmit the events of the Ominous Future to Genos after Saitama, by surprisingly turning back time, brought it along from the aforementioned future.

In a new upgrade, Genos can now use his core to expose a huge burst of blinding light. Genos tests this move out on Saitama during their second sparing mach. Saitama mentions that he has gotten brighter.

Trivia

 * In the Overwatch 2 One Punch Man event, Genji has Genos' core within his sword (between the hilt and the blade) for the Genos legendary character skin.