Continuity Rebooter

Sometimes, creators want to give their long-running series a fresh start, so they decide to make a Continuity Reboot. However, they sometimes don't want to just make a completely new reality for their series, instead making the reboot an actual part of the continuing storyline. In cases like this, one tool the creators have is to use a character to explain the reboot. Hence, the Continuity Rebooter.

A Continuity Rebooter is a character who, by some form of applied phlebotinum, causes either an Alternate Continuity to a work to be formed or the current continuity to be replaced with a new one. The character, whether intentionally or accidentally, changes his reality in such a way that the world becomes a fundamentally different place.

However, this trope doesn't refer to a character doing this as part of an Elseworld or What If? story, as those are non-canonical. It's also not any event which just lasts a little while and is eliminated with a Snap Back or the use of the Reset Button. A Continuity Rebooter must cause a long-lasting change in the series' continuity or create a long-lasting and well-explored alternate continuity to qualify, and that change must be part of the main canon. A series revolving around Time Travel doesn't count, since, well, that is a fundamental part of the plot(so no, Back to The Future isn't an example). However, a series that doesn't normally involve time travel and uses it as a device to change continuity would count if the change sticks.

Another factor in a character being a Continuity Rebooter is that the Continuity Reboot is NOT a complete one. The previous continuity is not wholly discarded, simply modified radically. In fact, a major plot point common to Continuity Rebooters is that the Rebooter remembers the previous reality. This also allows the series' creators to bring back fan-favorite characters and ideas from the previous reality to the new one, or even to bring the old reality back wholly(although never immediately). The new reality depends on the events of the previous one to exist, it's not invented wholecloth(like, say, an Ultimate Universe).

Basically, a character is a Continuity Rebooter if:
 * a Continuity Reboot or Alternate Universe is formed,
 * the reboot can be specifically attributed to the character's actions,
 * the previous continuity is not wholly discarded and the new reality depends on events from the previous one, and
 * the change sticks and is not immediately eliminated.

Usually, the character's mucking with the series' continuity is the plot behind a Crisis Crossover, and he uses the Timey-Wimey Ball or some kind of magic or cosmic plot device for the change. The character can alternatively be a Reality Warper who somehow changes his universe's events.

MASSIVE spoilers to follow:

Anime and Manga

 * Enrico Pucci in Stone Ocean, Part 6 of Jo Jos Bizarre Adventure. He reboots the universe, leading to Part 7, Steel Ball Run, which is set in the timeline of Part 1, but with drastically different characters and situations.

Comic Books

 * DC Comics has had a few of this kind of character, usually using a Crisis Crossover as the point for the changes to occur. The changes tend to be very long-lasting. Examples include:
 * The Anti-Monitor during Crisis on Infinite Earths, which led to the first true reboot of the DCU.
 * Hal Jordan during Zero Hour. Less extreme(and less well-received) than Crisis.
 * Hoo boy, Superboy Prime during Infinite Crisis, causing the second big reboot of the DCU and setting up MANY a later crisis crossover.
 * Barry Allen in Flashpoint, leading to the New 52.
 * Marvel has has a few characters of this stripe too, although the changes are nowhere near as wide-ranging as DC's and are usually limited to alternate continuities, with a few exceptions:
 * Legion in Age of Apocalypse.
 * Onslaught and Franklin Richards in Heroes Reborn.
 * Scarlet Witch in House of M, causing the Decimation of the mutants, which is the cause of many following events in the Marvel U.
 * Mephisto(to MANY a fan's consternation) in One More Day.

Film

 * Spock and Nero in the reboot of Star Trek. Spock's failure to prevent a massive Negative Space Wedgie from forming causes him and Nero to go back into the past, where Nero wreaks havoc with the timeline. Every change from the original continuity to the rebooted continuity is attributed to Nero's late-24th century Romulan cargo ship showing up in the early 23rd century.

Live Action TV

 * Dawn on Buffy the Vampire Slayer functions this way. She was introduced as a character in Season 5, and as soon as she appears, it's retconned that she's Buffy's 4-years-younger sister, and Buffy has always had a younger sister. Unusually, Dawn doesn't remember the alternate reality, and neither does anybody else (except possibly the people who cast the spell creating her). However, insane people see Dawn as an energy being, not a little girl--and this includes Dawn's mother, when her brain tumor starts making her irrational.

Video Games

 * Raiden in the newest Mortal Kombat game. At the end of Armageddon, Shao Kahn is triumphant, and the universe is headed for destruction. In a last act of godly influence, Raiden sends messages to his past self to try and avoid the Bad Future. This explains how the game is a remake of the first three games, yet a proper sequel at the same time.
 * Shu Shirakawa in Super Robot Wars Alpha Gaiden. His Neo Granzon causes the heroes to travel through time thousands of years, resulting in the world being a wasteland(as the heroes were not there in the past to save it).
 * Tatsuya in Persona 2. At the end of Innocent Sin, the world is basically on countdown to the apocalypse, and Philemon offers the main characters the chance to have their memories erased and time rewound to avoid it. However, Tatsuya, at the last second, relents on losing the memories of his comrades, and so the continuity of Eternal Punishment forms, in which Tatsuya is the only one who remembers what happened in Innocent Sin and Nyarlathotep has another chance at destroying the world.
 * Serge in Chrono Cross. When young, he was attacked by a panther, and at that moment the universe split in two: his current reality and another reality in which he died from the panther's attack. A big part of the game's plot is WHY he's so important to both universes that his near-death experience causes such a dissonance between them..
 * Lezard Valeth in Valkyrie Profile 2. You actually discover in the game that the Lezard you've been using is actually the previous game's Lezard, who traveled back in time to put his plan to make Lenneth his into motion from MUCH further back than in the original timeline, hence the game is already a reboot from the moment you start playing. Even Lenneth herself eventually appears and mentions how Lezard's causing changes in the timeline, and joins the past heroes in an attempt to stop him.