Thanos

Thanos is a character from the Marvel comics, and a competitor in the King for a Day Tournaments. He was initially selected as a popular fan submission, being #4 in the ballot overall.

King for a Day Tournament
Thanos was revealed as the eleventh competitor on July 21, 2018. His source list included rips from Marvel games, as well as rips related to "Fireflies" by Owl City and rips from . Thanos killed Ajit Pai in Match 6 of Round 1, but was ultimately defeated by Reggie in Match 3 of Round 2.

Quotes
Lines without a link come from the "All Victory Fanfares and VS Lines!" video.

King for Another Day Tournament
"With the newfound power of Disney behind him, Thanos re-enters the fight! After the previous tournament upset the balance of the universe, this Deviant once again seeks to restore it. Gauntlet on his fist and mouse cap on his head, the Mad Titan is prepared to win the tournament, and, of course, celebrate by going to Disneyland with those sweet half-length lines — there is only a 1 in 14,000,605 chance that he won’t!"

- Thanos' description on the MOJO!! Thanos returns in the King for Another Day Tournament. He now wears a Mickey Mouse costume, referencing Disney's ownership of Marvel. His source list has been expanded to include songs from mainline Disney canon and theme parks, music from Walt Disney Studios and Pixar films, and music from the Star Wars franchise. Fortnite music has been removed from his sources, likely due to his appearance in the game being deemed no longer relevant.

In Round 1, Thanos went up against Dr. Piccolo and lost, but in Round 1 of the Loser's Bracket, he defeated Pitbull and the Aliens. He faced off against Donkey Kong in Round 2 of the Loser's Bracket and was eliminated.

Quotes
Lines without a link come from the "All Vs/Victory Lines" video.

Trivia

 * Despite Fortnite-related rips being removed from Thanos' source list in the King for Another Day Tournament, the music for the tournament's reveal trailer still uses the "Dance Moves" Fortnite emote theme to represent him, likely due to his source list not being decided yet.
 * In King for Another Day, Thanos is the only non-faceless character whose face is not focused on during his VS. screen. Instead, the camera zooms into his crotch.
 * In his King for Another Day background art, there are a few small hidden Mickey shaped heads around him. The shadow also resembles this.

King for a Day

 * His victory quote against King Dedede references his line "You should've gone for the head." from Avengers: Infinity War.
 * His victory quote against Wario Bros. references the "Perfectly Balanced" meme said by him in Avengers: Infinity War.

King for Another Day

 * His vs. quote against Metal Ajit Pai references his loss against Thanos in the first round from the previous King for a Day Tournament.
 * His victory quote against Geno references the film series Star Wars.
 * His vs. quote against Off the Hook ft. Paruko references his line "I am inevitable." from the movie Avengers: Endgame.
 * His victory quote against "Weird Al" Yankovic references his line "All that for a drop of blood...", used as a meme usually replacing the word "blood".
 * His victory quote against Nintendo Power references a meme which replaces his line in Avengers: Infinity War "You should've gone for the head.".
 * His vs. quotes against King Dedede and Solid Snake are references to Thor and Captain America respectively. Specifically the encounters they had in Avengers: Infinity War.
 * His vs. quote against DJ Professor K references Thanos' love for "Death", which his love for her basically started the comic arc of Infinity Gauntlet. A reference to the Jet Set Radio Future song "The Concept of Love" is also made here.
 * His vs. quote against Quote references a message in Cave Story that appears when Quote falls or jump off the island, resulting in an instant death.
 * His victory quote against Quote is a reference to Thanos' adopted daughter, Nebula.
 * His vs. quote against Adam Levine references the Maroon 5 song "Misery".
 * His victory quote against Adam Levine references the Maroon 5 song "Maps".
 * His victory quote against Mr. Krabs references a scene from the SpongeBob SquarePants episode "Dying for Pie", where Mr. Krabs puts up his thoughts after SpongeBob took the bomb pie. At another point, SpongeBob answered that "watching the sunset with Squidward" is the last thing on the list, making Squidward thinks of what Mr. Krabs has said earlier, but also said that until sunset "the bomb will hit his lower intestine".
 * His vs. quote against Mariya Takeuchi is a reference to the popularity of her song "Plastic Love" on YouTube.
 * His victory quote against Dr. Robotnik references Toon Disney's airing of Adventures of Sonic the Hedgehog.
 * His vs. quote against Daft Punk ft. Pharrell is a reference to his army from the Avengers movies Avengers: Infinity War and Avengers: Endgame.
 * His victory quote against Daft Punk ft. Pharrell is a nod to his victory quote against Unregistered HyperCam 2 in the previous King for a Day tournament, in which he said the same thing.
 * His vs. quote against Papyrus indirectly references Papyrus' death in Undertale, where he was beheaded before dissolving into dust.
 * His vs. quote against Jack & Elmo references Main Street, U.S.A., a themed land inside the main entrance of the Disneyland parks as well as the quote "Gone, reduced to atoms." said by him in Avengers: Endgame.
 * His victory quote against Jack & Elmo is a reference to Avengers: Infinity War. In that movie, Thanos sacrifices someone while sadly saying "I'm sorry, little one..."
 * His vs. quote against Dr. Piccolo is a reference to Doctor Strange in the movie Avengers: Infinity War.
 * His victory quote against Dr. Piccolo references the Daily Dose meme.
 * His vs. quote against Jack Bros. references his line "Reality can be whatever I want." from the movie Avengers: Infinity War.
 * His quotes against Rhythm Masters references the song "It's a Small World".
 * His victory quote against MissingNo. references the arcade version of Thanos' ending in Marvel Super Heroes where all of the dialogue is taken from Magneto's ending due to a glitch.
 * His vs. quote against The Jazz Cats references the Flerken species, who are alien creatures resembling cats.
 * His victory quote against The Jazz Cats is a reference to jazz standard "Countdown".
 * His vs. quote against Eminem references his song "Rap God".
 * His victory quote against Eminem references the end of D12's "My Band".