Mary Sue
A Mary Sue is a pejorative name for a certain type of character.
Overview
Coined by Paula Smith in 1973 in her satirical Star Trek fanfic A Trekkie's Tale, the term "Mary Sue" has come to be used for an over-the-top badly written character that has broken the suspension of disbelief of a story to its detriment. The archetypical Mary Sue character is shallow and static, highly idealized, does not react to their situation in a way that makes narrative or psychological sense, and has traits that are extraordinary or out of place in the context of the story's setting. What exactly constitutes as "over-the-top" or "out of place" will of course vary from one setting to the next: a New Century Sue, for instance, might be a beautiful, intelligent, charismatic prodigy Zoid Warrior with a custom Ultimate X König Wolf, an Organoid and a gratuitous tragic backstory, who can beat Bit and Vega at the same time with her eyes closed, hogs the spotlight and takes agency away from the canon characters, reducing them to jobbers, worshippers or love interests.
Minutia
Most would agree on the broad definition outlined in the above section, but much controversy exists over the subjective finer details. The term "Mary Sue" is generally used among the fanfiction and RPG crowd, and some purists will insist the term only applies to fanfic and RP: others will argue the term applies to original fiction as well, pointing to characters such as Twilight's Bella Swan, Star Trek's Wesley Crusher or Ayla of Earth's Children as examples of Canon Sues. Mary Sues are often (but not always) surrogates for the author, and may share their personality, interests, political views, religious affiliation, sexual orientation, etc. A male Sue can be referred to as a Gary Stu, and although they certainly exist there is a regrettable tendency for them to not be criticized as harshly as their female counterparts. Anti-Sues are the extreme opposite of a typical Mary Sue - too flawed to be interesting instead of being too perfect, but still the recipient of undue focus.
Analysis
"Mary Sue" can be a loaded word: beware of those who use it as a shorthand for "any character I don't like who I think is too powerful or important." There is nothing inherently wrong with escapism or catharsis in fiction. Mary Sue is not a list of character traits: it is the disease, not the symptoms, and a character can have some Sue-like traits without necessarily being a Sue.
For example, Bit is coddled by the plot and has a special Zoid, but these traits are balanced out by his legitimate character flaws. Bit takes things for granted, is arrogant and impulsive, and a petty criminal to boot and the story frames these traits in a negative light. The crux of the story hinges on Bit being confronted with his hubris and learning from it. Bit is frequently the butt of insults and slapstick comedy at his expense, and the viewer is invited to laugh at Bit rather than with him. Bit does not stand on a pedestal above the rest of the characters: he's powerful but far from perfect, which helps to humanize him and make him a more relatable character.
Van Flyheight during Guardian Force is arguably a Canon Stu. Van transforms from a balanced character similar to Bit, to a mature, disciplined super-soldier with all of his character development happening off-screen between episodes 34 and 35. Guardian Force Van is a paragon of idealized masculinity: an excellent fighter in and out of the cockpit, and an inspirational leader hyped up by allies and villains alike (except for those who only exist to make him look better). Episode 52 is the closest the series comes to giving him an actual character flaw but quickly sweeps it under the rug in favour of making him even more badass; instead of actually growing as a character, Van just becomes more powerful.
