Back in the early days of My Hero Academia, there were several theories surrounding the doctor who told Deku he'd probably never develop a Quirk. Some believed that this seemingly inconsequential character was evil and working for All For One. Others believed that the doctor gig was a front so that he could find prospective Quirks that were worth stealing. The funny thing about these kinds of theories is how right they were.
Dr. Kyudai Garaki was eventually revealed to be the mad scientist everyone thought he was. Not only was he studying Quirks for All For One, but he was also responsible for supporting the League of Villains and, later, the Paranormal Liberation Front with his scientific expertise; in Spider-Man: Homecoming terms, he was their "guy in the chair." He may only be a supporting cast member, but his role throughout the MHA narrative cannot be understated.
Where Has Dr. Garaki Been Throughout My Hero Academia?
Dr. Garaki was the first to propose the Quirk Singularity Doomsday Theory approximately 70 years before the start of MHA. Unfortunately for him, the scientific community rejected his theory and he lost all credibility. However, All For One supported his theory and that's when the two of them became partners. As an act of gratitude, the doctor gave All For One his Life Force Quirk while keeping a copy of it for himself.
Besides learning how to copy and transfer Quirks through artificial means, Dr. Garaki also developed Nomus. These mindless slaves were made by taking the corpses of others and implanting multiple Quirks into them. They have been a serious threat throughout the series and have even given trouble to the likes of All Might, and they're all the product of one man.
At some point, Dr. Garaki founded Jaku General Hospital and other clinics, care homes, and orphanages focused on researching Quirk-based healthcare. Some fans theorized that he was also studying his patients' Quirks and looking for good ones to copy or steal; this led to the theory that he stole Deku's Quirk while examining him. However, all that's been confirmed so far is that this was all a front for his work for All For One and the development of the Nomus.
After All For One was incarcerated, Dr. Garaki was left in charge of a lot of the resources the two of them had accumulated over the years. This includes the labs, the stockpiled Quirks, the Nomus, and Gigantomachia. He was willing to share these resources with Shigaraki, but only if he proved himself to be a worthy successor to All For One.
What Is Dr. Garaki's Most Dangerous Creation?
After Shigaraki showed off his destructive potential, Dr. Garaki was glad to share everything he and All For One had gathered. He even let Shigaraki have the original version of One For All's eponymous Quirk. Whoever Shigaraki beats with his newfound power, that win can be attributed to Dr. Garaki.
As of the Paranormal Liberation War Arc, Dr. Garaki has been taken into custody by the Heroes, but it's too late. Not only did he transfer most of All For One into Shigaraki, but he also left the boy with some Nomus and even some Quirk erasure bullets. While he might not be able to provide any more help from his cell, the repercussions of what he's done will likely last the rest of the series.