After recovering enough to have a normal-ish daily life again, Rumi Usagimiya goes looking for an arm to replace the one she lost in her fight against several high-end Nomus.

She finds plenty of options in Khazan town, with even limb regrowth, parasytes and cybernetic replacements on the table. But the one which catches her interest the most is a sentient left arm, which also happens to be Lord Boros from One Punch Man. After a brief interview Rumi finds out that:

-Boros is already satisfied after fighting Saitama, so he's looking for something else to do to pass the time. Although he still wants to enjoy an intense fight every now and then.

-He still has quite a bit of power, but its limited to what can be contained in the arm. So although he can't use the Collapsing Star Roaring Cannon anymore, he can give Rumi energy projection and even a temporary boost via Meteoric Burst(though using it at 100% can shorten her lifespan and burn her out)

-No organ rejection! Assimilating the left arm of Lord Boros will not cause problems, and give Rumi regen similar to Deadpool's, but nowhere near Boros' almost-instant coming back from pulp and vapor.

-Psychic link: Lord Boros can share his decades of experience with Rumi and offer advice via a psychic link.

Also, since Boros has already faced the opponent he enjoyed fighting the most(even though it killed him...) he sees things a lot differently than Rumi. And considering his success until he met Saitama, he's got a lot to offer a young hero that's coming back to work after serious injuries. Rumi agrees, and several weeks later a new Mirko makes her appearance, with a black left arm(which turns white when using Meteoric Burst)

How does the MHAverse come to terms with the "new" Mirko?