It happens too quickly. As Seikyo assumes their position near Shigaraki, the man-child in question suddenly dashes away at top speeds. Following his trajectory, Seikyo sees the man in front of three students, a short kid with purple balls for hair, a girl with frog like features, and… Midoriya Izuku.
Shigaraki's hand is creeping towards the frog girl's face, intent on dusting her, but when he makes contact nothing happens. Turning to where the Nomu is, both Seikyo and Shigaraki find Eraserhead forcing his head off of the ground, glare sealing Shigaraki's Quirk.
Before anything else happens, Midoriya throws himself at Shigaraki, fist cocked back. As the attack flies forward, an intense wind pressure explodes from the boy, kicking up a large dust cloud. When the dust settles, Midoriya's face becomes overrun with terror as it's revealed Nomu took the attack, not even flinching from the overwhelming force.
A portal open directly in front of Midoriya's face, and Shigaraki's hand flies through to grab it. Before Seikyo can do anything to try and defend the boy, the USJ door fly off their hinges as a very particular Hero emerges from them. He stands tall and proud, but his usual winning smile is replaced with a look of fury that has most of the Villains shaking in their boots.
"Fear not, dear students," All Might walks forward, hand snaking upwards and tearing his tie off, "For I am here!"
Acting on base instincts, Seikyo erects a barrier around theirself as soon as they see the Symbol of Peace move. All Might blitzes through the area, knocking out most of the Villains and bringing the students and Eraserhead to safety in the blink of an eye, even sneaking in a hit at Shigaraki and Seikyo along the way. All Might's attack leaves a huge dent in Seikyo's barrier, the closest point dangerously close to poking into their neck.
As the barrier dissipates, Seikyo can't help but laugh. In an instant, All Might has turned the situation around, reigniting the hope of his students. Calling up another cavalry saber, Seikyo faces All Might, ghostly fire burning in their eyes.
"My oh my, All Might. You've certainly made this outing so much more interesting. Not just with your presence or actions, but with what you've dragged in with you as well."
All Might turns his gaze upon Seikyo, his fury tempered by curiosity, "And what do you mean by that, Villain? Cease your riddles at once, and repent for attacking my students!"
Seikyo laughs, "Oh my, feeling verbose today, aren't we? That's not your usual diction, All Might." The air suddenly takes on a suffocating aura, the smells of mold, decay, and damp earth playing at the edge of everyone's senses. "What I mean by what you've dragged in, is the seven lost souls clinging to you. Your everyday person is unlikely to even have one, but seven? It's just tickles my curiosity in a way I can't ignore. I know! Why don't we ask them?"
All Might's brow scrunches in confusion, "Pardon, souls? I'm afraid your speech makes little sense to me Villain, either explain yourself or fight."
Breaking down into hysterical cackling that sends a chill up even Kurogiri's spine, Seikyo reaches out with their power, and a ghostly form begins to construct itself. Kurogiri and Shigaraki have seen the usual spirits Seikyo calls upon when they deign to use the ability, faceless and nameless being made of pure energy at their beck and call, but this one appears nothing like them.
When fully formed she, for this spirit is unmistakably a woman, appears to have long black hair kept half up half down, pale skin, black eyes, and a noticeable mole on her chin. Her outfit is one of a Hero, a black bodysuit accompanied by a white cape, a white pair of boots, and yellow gloves.
Looking at this woman, Shigaraki feels a sense of familiarity tugging at the back of his mind, but All Might's expression drops into despair.
Looking over the woman, All Might looks ready to cry, "Master?"
The woman turns to All Might, confusion evident on her face, "Toshi? What's going on? Where am I?"
Seikyo clears their throat and approaches the woman, barely containing the urge to devolve into manic laughter and glee, one must be professional with the dead after all, "Pardon me Miss, but I believe I can explain. You're in the USJ, or Unforeseen Simulations Joint, a place constructed to train young Heroes in the ways of disaster rescues. If you don't mind me asking, who are you?"
The woman turns to face Seikyo, eyes narrowing slightly, "My name is Shimura Nana, now who are you and how did you bring me back from the dead?"