Of course he is a Mary Sue, because the true definition of Mary Sue is not simply just a perfect flawless character. In a more
correct definition, Mary Sue is a character that constantly breaks the rules of story-writing and focus balance of characters, such that the resulting character is always going over the top to the level of distorting the basic physics and fabrics of the story itself.
Being a self-insert of the author, or being absolutely perfect and powerful like no one else, or having countless abilities/skills/achievements before the start of story, or having the most beautiful/handsome appearance in the entire universe (according to story context), or having many boyfriends/girlfriends and still getting along with the new ones finely, etc. These are all common traits found in those characters accused to be Mary Sue, but nonetheless, these are in fact not requisites to be one. In order for a character to be deemed as Mary Sue, one should look at the interactions, relationship, and influence of the concerned character between other characters, the world itself, and most importantly the story plot. Also as an alternative Mary Sue index (according to my personal scaling), the amount of focus and attitude of the author towards the concerned character may also be accounted for the Mary Sue accusation.
To say it alternatively in the bluntest manner yet easiest to understand, is the author using characters as a tool to tell a dramatic story, or using a dramatic story plot as a tool to present a character? If it is the latter, then the concerned character will very probably be accused to be Mary Sue. Because the problem is, story plot became secondary, hence unimportant, thus author (or in anime's case, the scriptwriters) will tend to screw every other things without much reluctance and hesitation.
In order to suits the need of bringing the concerned character into focus, author will change the plot flow, all the important events will not happen until the concerned character is presented, the villains will not do their evil deeds if the concerned character is not here, the meteor that will destroy earth will just halt right above stratosphere if the concerned character is not here, the one-in-a-zillion super-great-ultra rare event will occur only if the concerned character is here. Then in order for the concerned character to be focused, all other characters are not to be focused, or used as a tool to contribute into the focus of the concerned character, thus the concerned character will be loved by others without reasons (or come up with some very cheap reasons) even he/she is a jerk with loathsome personalities and didn't do anything special to draw attention from others, all the others even the ones with professional occupations will have their IQ lowered, all the others will change their usual behaviors and minds when the concerned character is around, even wild Pokemon supposed to afraid of or attack approaching human will suddenly become more docile than a domesticated pet just because the concerned character is around, and of course all the sidekicks are then just downgraded to be supporting characters or at worst henchmen. And then in order for the concerned character to be the absolute focus of the show, he/she will then be the only one allowed to accomplish something important, he/she is the only one to save the world and save the day, despite that he/she have reliable and powerful friends, they will not move at that critical moment, but only to sit back waiting for the concerned character to be return like a hero to be praise, or offer help but "sacrificed" at the very early stage.
Please note, having over-the-top background settings such as being a super genius of Alakazam IQ, or superpower equivalent to Saiyan despite being a normal earthling, having countless achievements, being a multi-skilled all-round worker, having complicated/tragic/affluent background like no one else, being a half-breed or mixed-breed or even a non-earthling or non-terrestrial being, or having abnormal coloured hair or eyes or skin colour, etc. Surely these traits are rather eye-catchy, so by the very first glance reader/audience will focus on characters with such traits. But still just like I claimed, that is not requisite to be accused as Mary Sue. The most critical point is, are these background settings and traits form part of the overall story? Will these things attribute to future story building? Will these things attribute to actions/reactions, behaviour and mindset of the concerned character? If the answer is all "No", where author just add these traits to the concerned character for the sake of so-called coolness, or maybe author just add in such abnormal traits right in the middle of story for some once-off events but originally didn't included in the usual character design, then such character will be very likely be accused to be Mary Sue.
