WARNING: The following contains spoilers for My Hero Academia Chapter 343, "Let You Down” by Horikoshi Kohei, Caleb Cook, and John Hunt, available now in English from Viz Media.
After years of scheming in the shadows, All For One finally made his most offensive move yet in My Hero Academia when he seemingly cornered Midoriya Izuku. Unbeknownst to him, the Pro-Heroes had baited him into an ambush and quickly rallied to Midoriya's side with the help of the series' most tragic and mysterious Quirk, Warp Gate.
Warp Gate belonged to Kurogiri, Shigaraki Tomura's mysterious guardian and protector. Using it, Kurogiri could create shadowy portals that teleported himself or anyone who passed through them to his desired location. It's a highly useful Quirk that was a thorn in the heroes’ side early on in MHA, but its origin hid a scandalous secret; Warp Gate was a synthetic Quirk created through experimentation by All For One and Dr. Garaki, and Kurogiri was actually a Nomu.
The Quirk that formed Warp Gate's base was Cloud, which originally belonged to Oboro Shirakumo, an old friend of Present Mic's and Eraserhead’s. Shirakumo's consciousness was almost completely erased by the process that turned him into a Nomu, and he's been mostly unresponsive since the heroes captured him. His current circumstances make it unlikely that he's aiding the heroes with Warp Gate of his own volition, but the possibility does exist no matter how small.
It's more likely that Warp Gate was copied and activated by Class 1-B's Monoma Neito. Monoma's Quirk, Copy, allows him to freely use the Quirks of others he's touched for up to 10 minutes. Certain Quirks don't work exactly as advertised when Monoma copies them and it’s unclear how Warp Gate's synthetic nature would affect Monoma's Copy but for now, this seems the best explanation for how the Pro-Heroes got their hands on Warp Gate. A mysterious hero bearing Monoma's likeness was also shown emerging from a portal which lends further credence to this theory.
Warp Gate wasn't the only surprise this chapter had in store as it featured the highly anticipated return of everyone's favorite Rabbit Hero, Mirko. She was last shown during the Paranormal Liberation War, where she'd suffered heavy injuries after defeating an entire squad of High-End Nomu almost single-handedly. Mirko looked to be back in fighting shape now, and she's sporting a dangerous-looking support item; a prosthetic arm with attachments that don't look like they'll be much fun for any villain caught on the wrong end of it.
Mirko was joined by several other Pro-Heroes in her grand reappearance. She made her entrance alongside all-stars such as Gang Orca, Wash, Endeavor, Edgeshot, and Fat Gum. This group of heroes, sans Endeavor, were carefully selected to deceive Ragdoll's, Search Quirk. None of them directly engaged Shigaraki in the previous war and as such All For One had no way of knowing they were arriving so suddenly. Search is any tactician's dream Quirk as it also gives its user information on the weaknesses of those it's turned upon. Knowing All For One, he's probably already planned contingencies for the heroes that first confronted him. With this almost entirely new batch, all those plans are out the window.
