Izuku's quirk is Tonberry. He is a human version of a Tonberry with all it's effects from Final Fantasy now within MHA.
In this scenario Izuku awakened a quirk, however he looked the same only his green pupil's turned yellow. As Izuku features although he was human, began to look softer almost chibi in his features as he's noted to be very adorable.
However Izuku quirk made him indestructible, every injury that tried to hurt him couldn't leave even a scratch in his skin. Weapons that made contact don't even leave a scratch upon him. Since Tonberries are canonically immune to Death, Poison, Stats being lowered, Petrify, Darkness, Silence, Berserk, Zombie, Sleep, Haste, Slow, Stop, Reflect, and Doom.
Izuku is utterly immune to Tomura's decay since it's basically petrification/instant death.
Izuku doesn't need to sleep to function, as he sleeps for pleasure.
Izuku is immune to poison, as he doesn't get food poisoning nor has he been sick his whole life.
Izuku can't be blinded since his immunity makes it impossible for his vision to be hindered.
Izuku is immune to being stopped by other quirks that prevent movement.
Quirk suppression cuffs or even quirk's that stop other quirks don't work upon him since it's basically a quirk version of silence.
However Izuku's most dangerous power is 'Everyone's grudge.'
Where Izuku calls forth his lantern, and can instantly one shot opponents depending on how many people or things they've hurt where you will feel every opponent you've ever hurt being redirected at you till you're comatose or worse dead. However if used upon those who have killed, the victim instantly dies.
The downside is that like a tonberry, Izuku isn't fast as he's normal human speed. Though if he goes under a quirk awakening than he gains the power of a king tonberry becoming even more durable, gaining the power to teleport, teleport/warp numerous things upon others, and call forth smaller regular final fantasy Tonberries as heralds.