Attack of the Gigantic Speedreading Faggot 7493857: The Reckoning.
Not only do you have a piss poor grasp of what a Mary Sue is, you're complaining about an unrealistically over-effective character in fucking Made in Abyss, the manga where a fucking twelve year old girl, an eccentric and encyclopedic genius on the abyss, makes it through the most dangerous chasm on the planet in which countless older and more experienced delvers than her met their doom. She not only survives but becomes the youngest white whistle in recorded history, and is adored by her boyfriend/personal relic who's obsessed with her to the point of valuing her life over his, and also her bunny/surgeon/orphan/narehate/murderer/onahole/bff whos affection borders on a warm lesbo crush.
Speaking of unrealistically perfect, Nanachi's the most popular character and is seen by the vast majority of readers, much like yourself no doubt, to be the character who saved the manga. This is despite she's a twelve-ish orphan genius loli rabbit with a dark and tragic backstory, who knows how to use medicines and poisons and is apparently a surgeon, who was "forced" to murder innocent children and delvers but not only got completely absolved for it but adored all the more by her bestest best friends forever and is constantly showered with physical affection from both of them. This is the most popular character, again I remind you.
I also notice people jerk off to Lyza relentlessly, worshiping her like a goddess and cursing her daughter's "potato genes" even though she's the biggest most sparkly perfect Mary Sue of them all. She's the most famous white whistle, even though she's basically just a Disney princess with a pickaxe.
If you honestly looked over this whole cast of literal perfects and singled out Reg specifically as a Mary Sue amongst the lot of them without picking up a hint of the irony then you have my condolences for whatever head injuries or cranial defects you have clearly suffered.