The Last Human

The Last Human is an Adventure Time fanfiction created by Ruby Sword.

After Princess Bubblegum had an unexpected marriage, a 17-year old Finn begins to discover photos of the past when humans were still around. This motivates him to find out what had happened to them all. As he investigates, along with the help of Jake the Dog and Marceline the Vampire Queen, answers begin to unravel and he starts to understand their history and what wiped them all out.

Tropes:

 * After the End: To an even greater extent than the original series. Marceline eventually directly tells Finn how the Mushroom War led to the creation of Ooo, as in this continuity she lived through the Mushroom War herself. Apparently the bombs blew open the dimensional barriers, bringing back the magic, and that caused the mutation of the remaining life-forms into the bizarre mutants we know and love.
 * Alternate Universe: The earlier chapters were written prior to the begining of the third season, so several things are quite different in this continuity. Among other things, Bubblegum was never aged back up, and Marceline originally looked human before she was vampirized.
 * Arranged Marriage: One kicks off the very begining. Bubblegum has one set up by her dad and married to the Ice Prince, who has supposedly rescued him, and does so to unite the Ice and Candy Kingdoms.
 * Birds of a Feather: Finn and Marceline's many similarities lead them into a synergistic relationship that quickly incorporates a lot of WAFF that eventually goes from the True Companions they are in canon to a Relationship Upgrade.
 * Cute Monster Girl: Marceline, from Finn's point of view.
 * Friendly Neighborhood Vampire: In spite of her borderline Chaotic Neutral attitude, Marceline is revealed to be this. Billy mentions that there are other vampires extant through Ooo that feeds on the blood of random things to make up for being unable to feed on humans. When Finn and Jake mention Marceline and her ability to drain the color red instead, Billy remarks how unusual that is and suggests that she's much more moral and humane that she lets on.
 * Good Feels Good: Marceline states that this is part of her attraction to Finn.
 * Hair of Gold: Finn, of course. Marceline's attracted pretty hardcore by it.
 * Hurting Hero: Finn is this after his hopes for a relationship with Bubblegum are dashed when she has an arranged marriage that's (from his perspective, due to being locked out of the loop) comes from nowhere. Marceline picks up on it and holds a party to make him feel better, telling people that it's a very late birthday party.  The author has stated that this is pretty much her theme throughout the story.
 * Kiss of the Vampire: Implied in a dream sequence Finn has at one point where Marceline drains him, with his consent.
 * Knight Templar:
 * Last of His Kind: Much of the story deals with Finn investigating the decline of his species and the circumstances thereof.
 * Mayfly-December Romance: Half this fic is about the circumstances of Finn possibly being the last human. The rest of it is all about Finn and Marceline's growing relationship, and indeed that's what's generally more popular with the reviewers.
 * Older Than They Look: As expected, Marceline, and in this story she's old enough to have been born before the Great Mushroom War.
 * Relationship Upgrade: Specifically, Finn and Marceline upgrade from True Companions to lovers, and the plot revolves around the circumstances of it.
 * He Is All Grown Up: Marceline has this reaction to Finn earlier on when she realizes he's a little taller than her now, and more profoundly when ever they've started to express romantic interest; she takes off his hat and is shocked at his handsomeness.
 * Ship Sinking: Fubblegum is sunk by the plot right at the beginning of the story; she gets married to the Ice Prince, a newcomer to Ooo who's spent some time abroad, and the earlier parts of the story deal with Finn suffering a broken heart from this before growing closer to Marceline. Marceline being who she is, she throws a party to celebrate him getting over her.
 * Smug Snake:
 * The Ladette: Marceline, as noted by Finn. And it appears to be a good point for him.
 * Finn: I don't see her wearing an apron and and being all wifey, you know?
 * The Paragon: Finn is very much this, espicially from Marceline's point of view. It totally floors him.
 * Vegetarian Vampire: While in canon it's assumed that vampires CAN be like this, it's established here that vampires normally go around killing for sustenance. Marceline's fairly unique for feeding on color instead of killing to survive, and this revelation fuels Finn's insistence that she's a better person than she lets on.
 * You Never Asked:
 * An inversion; Billy assumes that humans are extinct until Finn explains this to him, and he explains that he never told Finn about a group of humans he once met because Finn hadn't explained that he was human.
 * On a more serious front,
 * Who Wants to Live Forever?: While not a particularly prevalent theme in-story, as Marceline's attitude seems to lean more towards Living Forever Is Awesome, a rather nasty moment from her is revealed to be the result of her father trying to convince her that trying to romance Finn is a bad idea, alluding to his own time with her mother. She and Finn make up, but she implies a fair bit that her inevitable outliving of Finn and their friends is a source of a lot of grief for her.
 * Vegetarian Vampire: While in canon it's assumed that vampires CAN be like this, it's established here that vampires normally go around killing for sustenance. Marceline's fairly unique for feeding on color instead of killing to survive, and this revelation fuels Finn's insistence that she's a better person than she lets on.
 * You Never Asked:
 * An inversion; Billy assumes that humans are extinct until Finn explains this to him, and he explains that he never told Finn about a group of humans he once met because Finn hadn't explained that he was human.
 * On a more serious front,
 * Who Wants to Live Forever?: While not a particularly prevalent theme in-story, as Marceline's attitude seems to lean more towards Living Forever Is Awesome, a rather nasty moment from her is revealed to be the result of her father trying to convince her that trying to romance Finn is a bad idea, alluding to his own time with her mother. She and Finn make up, but she implies a fair bit that her inevitable outliving of Finn and their friends is a source of a lot of grief for her.