The term "Mary Sue" gets thrown around a lot in the anime community - but what does it actually mean? While the real definition is controversial, there are several basic traits that most people agree on. A Mary Sue is an overpowered character whose abilities either break the established rules of the series in question or are so ridiculously OP that no series could possibly account for it. What's more, everyone in the series adores this character, even if it's not clear why that's the case. They often have multiple love interests or have other characters salivating over them. Their personalities are typically flawless, and when they do have flaws those flaws are endearing and don't really have a negative impact on their lives. A male version of the same thing is called a Gary Stu.
A character being a Mary Sue or Gary Stu isn't necessarily a bad thing - there's a reason the archetype exists and that is wish fulfillment. Sometimes you just want to watch your favorite anime guy or gal kick butt and you don't want to worry about the details. But sometimes Gary Stu and Mary Sue anime characters can fall flat, because they don't always face the kinds of challenges that a character struggling with personal flaws and weaknesses might face.
Vote up the characters that you think qualify as Sues and Stus, and vote down the ones you think are miscategorized.
When anime fans talk about canon Sues and Stus, Kirito is on pretty much every list. Why? Because he's ridiculously overpowered to the point where his fights feel like Deus ex Machinas, and every girl in the series is at least a little bit in love with him. How did he get ahold of admin powers at the end of the series? Why does everyone, including his adoptive sister, have the hots for him? Because he's Kirito!
He does recieve a little more development in Sword Art Online II - it's hard to call the version of Kirito who is dealing with PTSD-like symptoms over his time in a virtual world a Stu. But in the first series, the term fits.
Shin Walford is a relatively new addition to the world of anime - and to the world of Gary Stus. He's transported into another world and reincarnated as a baby, then adopted by a mage who teaches him everything he knows. Naturally, Shin excels at every form of magic he learns, blowing his guardian's expectations out of the water. His only flaw is that he never learned to interact with society outside of his relatively secluded home, but this doesn't hamper him in any significant way.
Tatsuya Shiba supposedly has a major disadvantage in the world of the National Magic University Affiliated First High School. Despite being born with no magical abilities, he ends up becoming one of most powerful people around through a variety of convoluted circumstances that we won't get into here.
What makes him a Gary Stu rather than just being OP? His abilities, name, equipment, and other traits are all based on Shiva, a god from Hindu mythology. If a creator intentionally engineered parallels to an actual deity from a real world religion, chances are high that the character is a Stu.
Make no mistake - Rimuru Tempest is freakin' awesome. Watching him solve every single problem with diplomacy and kindness is a delight. But while Rimuru is an example of a Gary Stu done right, he still is very much a Gary Stu.
He begins the series as a lowly slime monster, but quickly realizes that he isn't just powerful, he's the most powerful being in his new world. Seriously, by absorbing his enemies he can master any ability he wants. While people underestimate him at first, everyone who meets him realizes how hard he rocks, and willingly dedicate their lives to helping him out.