Who (Most Likely) Trained Himiko Toga in Stealth & Close-Quarters Combat?
Short answer: We never get an explicit answer in the manga, and Horikoshi deliberately left it as one of the small “aborted threads” from earlier arcs. But based on everything shown and stated, here is the ranked likelihood of who taught her (or contributed the most):
Self-Taught + Natural Talent (Most Canon-Compliant Answer)
Toga repeatedly says she learned to “erase her presence” and move unseen while she was on the run after her middle-school massacre.
She explicitly tells Curious (Ch. 226) and later Ochaco (Ch. 341 & 391) that after she snapped and attacked Saito, she spent years evading police and heroes by herself.
Quote: “Day after day, the police and heroes came after me… so I became sensitive to how other people behave… and I learned that the world treats high school girls just a little bit kinder. So I don’t get caught. I don’t get caught. I don’t get caught.” → This is presented as the primary origin of her stealth mastery. She is a natural predator who refined her skills through real-world survival, not formal training.
Some Informal Guidance from Twice (Secondary Influence)
Twice is the League member Toga spent the most time with on-screen.
Twice himself is extremely skilled at misdirection, cloning decoys, and moving unnoticed (he infiltrated the Yakuza base twice with Toga).
Toga copies a lot of his speech patterns and playful behavior, and they are constantly together in every arc after Kamino.
It’s very plausible he gave her tips and they practiced together, even if it was never shown.
Very Minor Input from the Rest of the Early League (Unlikely to be the Main Source)
Dabi → Zero stealth, all flashy fire. No.
Spinner → Decent driver and fighter, but no stealth feats.
Mr. Compress → Excellent sleight-of-hand and escape artist, but his style is theatrical, not silent infiltration.
Magne → Dead before Toga could train extensively with her.
Muscular, Moonfish, Mustard, Chainsaw Nomu → All brainless muscle or one-trick psychos. Absolutely zero evidence any of them ever trained her.
An Unknown Mentor Pre-League (Pure Speculation, But Popular Fan Theory)
Some fans believe she may have briefly fallen in with another underground figure (possibly tied to the Creature Rejection Clan or some minor yakuza group) before meeting Stain’s ideology and seeking out the League.
There’s zero canon support for this, but it would neatly explain why her close-combat and stealth skills are so much higher than almost every other high-school-aged character.
Final Verdict
Canon answer: Toga is self-taught through years of evading capture after her first kill, with possible informal tips from Twice later on. Everything else (Dabi, Muscular, etc.) is flatly contradicted by what we see of their personalities and skill sets.
The “aborted arc” you noticed was real — Horikoshi clearly planned to reveal something about her past training around the License Exam / Shie Hassaikai period, but ultimately decided her tragic “learned it to survive while being hunted” backstory was stronger and dropped the idea of a specific mentor.
So in the end, the scariest answer is the canonical one: a 15-17-year-old girl became that lethal completely on her own, because society left her no other choice.