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I apologize for not writing any new chapters of Quirkless REBOOT for a good long while now, my depression has been sucking the energy from me and I was not in a very good mind set, I will be taking a break from this fic and hopefully coming back to it once I am in a better place mentally, as an apology for making y'all wait so long here's an explanation of Izuku's quirk and the current quirks he possesses due to the vestige's rebellion against Ass Might.
Izuku's quirk.
Name: Reboot.
Classification: Reboot is an energy accumulation type/mutant type(due to its effects on Izuku's cells) and emmiter/transformation type.
Description: Reboot sits in the background gathering energy and storing it in Izuku's cells waiting to be triggered. It can be triggered by lethal damage causing Izuku's heart to stop. Upon which the quirk reaches out for any quirk energy (it does this by detecting alpha plus brainwaves due to the quirk gene, copying the quirk gene it finds and storing it for later use by Izuku. Conscious effort on Izuku's part will result in being able to use one of these quirks at random. However now the vestiges who left OFA have joined into his quirk, they can sense which quirk he wants to active and help him do it, he currently has access to the vestiges quirk but has not yet used them.
(The vestiges basically help streamline Reboot's healing and quirk activating process and effectiveness. Izuku could possibly train his quirk to this level without them being there but I think he deserves a little help don't you? )
Izuku can survive having essentially multiple quirks and survive lethal damage and his heart stopping by the changes to his cells his quirk made when manifesting, turning them all into slightly specialised STEM cells. When lethal damage is taken and Izuku's heart stops his quirk does a couple of things.
1) It pulls Izuku's consciousness into 'storage' with the other vestiges and its self, to protect its host while it heals Izuku's body.
2) Using the energy it has gathered since the time of last activation it ramps up STEM cell production and forces them around the body to any damages areas. Once it has healed all damage it begins to pull in oxygen and make the heart beat oxygenating the body.
3) It begins to pull in any energy it can find, heat, kinetic, potential and any quirk genes it can find in an effort to recharge and adapt to what killed its host to hopefully avoid lethal damage next time.
4) When the body is healed and the quirk is recharged (it does this very quickly) it pulls Izuku's consciousness from 'storage' and puts him back in the 'drivers seat' of his body as he wakes up, healed and in no pain.
Izuku's body can handle the damage multiple quirks have on him because the quirks are not being used untill Izuku decides to activate the dormant quirk genes in 'storage'. If tested for quirk genes doctors would only find one set of quirk genes if Izuku is not activating any quirks at that moment.
Of course if Izuku activates more than one very rapidly it would cause quite a bit of strain. Like a work out where you strain your body and it heals back to be stronger, the same would apply to Izuku's quirk tolerance, as he uses more quirks faster and for longer, his quirk would be able to more efficiently heal the damage and strain that causes him.
Please take this as an apology that I am not currently planning to write Quirkless REBOOT for a while. I hope you enjoy the insight into Izuku's quirk.