It's been nearly ten years since Izuku manifested his quirk. In that time he's figured out its main mechanics and limitations.
Izuku's quirk, Summon, allows him to open portals to another world and pull creatures through it. These creatures have been labeled Pokemon by the inhabitants of that world. He can only call Pokemon of a certain power threshold or lower to his world, and although stronger Pokemon are able to cross the portal themselves, this exhausts Izuku. Humans are unable to enter the portals at all.
Izuku is able to form a bond with willing Pokemon that allows him to call them to his side for a much lower cost, even if they would normally be out of his power threshold, and from his world as well.
Izuku's day has been more or less normal so far. His peers taunting him about a "villainous" quirk was bad enough, but then he gets sidetracked on the way home. Apparently his mother's Eevee got free and was following him again. As he was walking home with Eevee he's assaulted by a living pile of sludge that leaves a large part of him wondering when he called a Grimer or Muk.
Instinctively Izuku Calls for a Pokemon, any Pokemon to help him. Eevee answers, of course, but no matter what move she uses she can't seem to affect the sludge being.
Izuku Calls again, desperately hoping for something, anything to come and help him as the edges of his vision start fading to black. The last thing he sees before he passes out is a tall white bird coming charging through the black-blue portal his quirk uses.
Izuku cracks his eyes open, hissing at the light that assaults him. The huge white bird is standing between a skeletal looking man and him and Eevee, hissing at him with its (her), her swings spread wide.
"Young Man, are you alright? I would perform a medical exam but this goose isn't letting me close." Upon being mentioned, Swanna hisses at the skeletal man again.
"I'm alright sir, but I'm going to get myself checked out in a hospital later, just in case." As he speaks Eevee makes a grumbling sound to which Swanna responds with a high trill. "Oh, and Swanna says that she's going to, and I quote "Keep his destructive ass out of the grave.""
The skeletal man splutters in surprise and mild offense as the Swanna checks Izuku over and croons at him. Then she walks over to the skeletal man and starts to peck at him until they are moving away from Izuku.