Guardian of the Multiverse

Our universe is not the only one. In fact, it's just one of countless Alternate Universes in The Multiverse. The Multiverse is so big and complicated, that it needs a being capable of watching over all of it. Enter the Guardian of the Multiverse. He prefers to be The Watcher, to observe and not interfere unless it's necessary. However, if your Evil Twin escapes his native universe bent on attacking yours, a Galactic Conqueror, Dimension Lord, or Omnicidal Maniac wants to break the barrier between realities, an Eldritch Abomination tries to spread its horror through the dimensions, or if two alternate worlds declare a war that threatens multiple realities, the Guardian of the Multiverse will be the Deus Ex Machina called upon to stop it. If changes in one world's history will endanger others, he will gather a team to Set Right What Once Went Wrong and become their Mission Control. And if a Multiversal Conqueror rises to enslave all worlds in all galaxies of all dimensions, the Guardian of the Multiverse will gather an army needed to stop him (or be the very first person he kills, because of The Worf Effect).

He's often an Original Generation in a Crisis Crossover. Likely to be the Big Good of a setting.

He's one of the two highest-ranking Authority Tropes, the other being Multiversal Conqueror. The next rank down is Dimension Lord.

Anime and Manga

 * The Lord of Nightmares in Slayers is this. She created four worlds, put in a god and devil in each one and if called upon can undo anything in any of those worlds that she sees fit to.
 * CLAMP has Yuuko Ichihara of xxxHolic as well as  in Magic Knight Rayearth

Comic Books

 * DC Comics had The Monitor before Crisis on Infinite Earths and Monitors after Infinite Crisis. In Final Crisis  In summary, they are really bad at this.
 * This is really messed up in Marvel Comics. The Watchers sometimes do it, The Timekeepers were doing it in their own sick ways, and The Time Variance Authority once was also protecting the Multiverse like it was its duty. Then The Exiles came, and turned everything upside down. First we get a mechanism that takes the form of The Timebroker - combined minds of people gathered in order to save The Multiverse. Then a bunch of bug aliens becomes its "man behind the man". And then some godlike couple pops out of nowhere and quickly disappears, to be replaced by a good, female version of Kang.
 * And Man-Thing is the guardian of the Nexus of All Realities.
 * It should be noted that -with the exception of Man-Thing- all of these characters are self-appointed guardians. They debate even with each other who has the right to the position.
 * Generally speaking though the Living Tribunal generally performs this function, being second only to The One Above All; IE Kirby/God from the above, who rarely makes appearances.
 * In the Marvel UK books the role is taken by Merlin and Roma of Otherworld.
 * Access from the Marvel Comics vs DC Comics series. He can't even stay in either one for too long or else his power will start warping the realities.
 * Ringo from The Crossoverlord.
 * The comics version of Sonic the Hedgehog is destined to be the hero of all possible realities, according to Zonic the Zone Cop.
 * The short-lived 2000 AD series Indigo Prime featured an agency whose job was to clear up the rifts in space and time left by careless time travellers and universe jumpers.
 * WITCH has The Oracle, the Guardians and everyone else that is part of Kandrakar.
 * Sinister Dexter introduced Context, a group of brightly-coloured ninjas with this role, in 2011's "Apocalypse Shtick" storyline. Since any action Context takes directly will harm the universe they're in, they prefer to hire local talent.

Film

 * The One has the Multiverse Agency in the Alpha Universe, the only universe to develop interdimensional travel. The MVA was created to control travel between the 'verses, mostly from the people of the Alpha Universe.

Literature

 * The Magids in Diana Wynne Jones' Deep Secret. Chrestomanci fits this as well.
 * Robert A. Heinlein's later novels have the protagonists forming a group with the goal of protecting the continuity of The Multiverse, after the events in The Number of the Beast unlock pan-dimensional travel. It's implied that they are fighting not only against other dimension-hopping groups with nefarious intentions, but possibly even other authors. Considering that Heinlein is in fact the author of the novels, it's also an example of Leaning on the Fourth Wall.
 * In Michael Moorcock 's multiverse, this role is filled by an entity called the Cosmic Balance, which is all about maintaining the Balance between Law and Chaos. It's not entirely clear whether the Balance is sentient in the same way as humans and gods, but it fits the trope either way.

Tabletop Games

 * Chaos, a Tabletop RPG, gives us the Devotus Chronos. A highly elite, highly trained, highly powerful, and highly secretive branch of the Omniversal Union. They are given a variety of gravely important tasks to fulfill, but what it basically boils down to is this: they're space cops and Time Cops whose job is to act as scouts and spies in multiverses yet to be Raptured into The Union, to deal with serious threats to omniversal security that arise in said universes and/or time periods, and basically to keep all creation from going to hell in a handbasket (as much as possible).

Video Games

 * The little-known PC game The Siege of Avalon Anthology has the Astral Guardian, who watches over the planes of existence. She gets locked in a magical flaming prison until the player breaks her out, and all sorts of trouble occurs in the meantime.
 * Fanon usually places Master Hand in this role, since there's not much else to explain his existence in the Super Smash Bros universe.
 * The Longest Journey series features the Guardians of the Balance, whose job is to make sure that magic doesn't get into Stark and science doesn't slip into Arcadia, the two major worlds in that multiverse. Each Guardian serves for 1000 years before a new one is found in either world and sent to Guardian's Realm. It should be noted, however, that this is only a temporary measure, as the two worlds are expected to be rejoined in the indeterminate future when humans are finally ready to responsibly use both magic and science.
 * Also, the Guardian is not actually expected to fight anyone. They have the Sentinels for that.
 * Neue Einst Regisseur from Super Robot Wars is the Eldritch Abomination variety, or at least believes this to be his title, having existed since the beginning of time with no other purpose. He/it/they decides to eliminate mankind, whose consciousness causes the entropy of universes, but also hints at trying to stop someone from retrieving the "twelve keys to the ancient records" which are scattered across The Multiverse. However, his power to cross dimensions is limited by the same rules as anyone else, suggesting that he's nothing more than an alien who decided to butt into the multiverse's business and got his ass kicked for his troubles.

Web Originals

 * This is essentially what the Protectors of the Plot Continuum do.

Western Animation

 * Vector Prime in Transformers, under the command of Primus.
 * There's a good chunk of them, all with different jobs; Primus is the guy who keeps Unicron from destroying the Multiverse, Vector Prime is the Guardian of Time, Nexus Prime is the guardian of Rarified Energon (used to create new Transformers) and keeper of fellow Firstforge Prima's Star Saber, and The Fallen used to be the caretaker of the end of the universe ... depending on which continuity family you're currently following.