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Heroes tend to lead very... interesting lives, full of so many strange and fantastic sights and events that before long he's Seen It All, or at least dealt extensively with one type of non-mundane creature/phenomenon. As a result of this, or coincidentally, the character now casually lives alongside or in close proximity with an alien, ghost, vampire, mutant and/or robot (though not necessarily all in one character).
The nature of the arrangement varies. They might be allies of long standing, the monster came with the apartment as an unadvertised "bonus" and they've since hit it off, or both have made the completely mundane arrangement to share living space. It doesn't even have to be The Unmasked World, any monster that can fool muggles is going to have to pay the rentnote , and it's sometimes convenient to share space and bills. If they're Vitriolic Best Buds, expect them to claim they keep the other as a "pet".
The setting doesn't have to be a Fantasy Kitchen Sink for this trope to happen, it just needs one difference that makes it Like Reality Unless Noted, and the character has to live right next to that thing. For maximum effect however, the character's house might become a full blown microcosm of the world's supernatural scene.
The "roomie" might be a Nice Guy ghost, a personable Fully-Embraced Fiend of a vampire, The Protagonist's Magical Girlfriend, or one of the Mad Scientist's Mascot Mooks that has been reprogrammed/retrained. If Tom develops an Unwanted Harem, each girl will be from a different monster race. Mind the Fur Against Fang.
- Doraemon: The title robot cat lives in Nobita's closet (one of the earlier examples).
- Urusei Yatsura: Ataru has an oni living in his closet (possibly a pun on the Japanese slang for Pretty Freeloader).
- In Bleach, Shinigami Rukia lives in Ichigo's closet (much to his displeasure).
- In Mahou Sensei Negima!, Kotaro starts staying with Natsumi and Chizuru after a little incident that resulted in amnesia. He gets his memory back and has been staying with them ever since (even using Natsumi's surname so he can pass as her "little brother". Considering they're well on the way to Official Couple, one has to wonder what that implies.)
- The plot of Magical Girl Lyrical Nanoha A's begins when the four Ridiculously Human Alien Program Wolkenritter appear before Hayate and start living with her.
- Brigadoon: Marin and Melan - Melan, a flying alien cyborg with weapons for hands, moves in with Marin and her human family in order to protect her from other aliens that try to kill her. He is treated as a member of the family by everyone in the house.
- In Death Note, Ryuk, a Shinigami, lives in Light's room, and describes himself as being his roommate.
- In Daily Life with Monster Girl, Kimihito has a whole harem of monster girls living in his house.
- The main premise of Kamisama Kiss is an Ordinary High-School Student moving in with a Kitsune and a pair of shrine spirits.
- Gugure! Kokkuri-san features an Emotionless Girl living with a kitsune, a tanuki, and a dog youkai.
- The premise of Gingitsune: a shrine family lives alongside the shrine's resident kitsune, although only the heir to the shrine can see him.
- Takeo-chan Bukkairoku is about a supernaturally unlucky girl who moves into a youkai apartment house.
- Miss Kobayashi's Dragon Maid has several cases of dragons living with humans, though the only one who's actually referred to as a roommate is Fafnir. Averted with Elma, who is the only one to have her own apartment.
- Elegant Yokai Apartment Life: has this as its main premise and setting: After the school dorm burns down, the main protagonist Yushi is left with nowhere to turn to, except into an apartment filled with all kinds of Yokai and other odd beings (and even some ghosts) from the illustrious Japanese folklore.
- Mononoke Sharing is about a high school student moving into an apartment with five Yokai living in it. The main reason she was allowed to move in was part of a sort of social experiment to see if Yokai are capable of integrating into human society.
- The Caged Demonwolf in Empowered, Sealed Evil in a Can that can still see, hear and talk, and spends most of its time snarking at its captors and perving over their sex lives.
- The very premise of Monster Allergy: the protagonist Zick is a boy that lives in a Detention Oasis for monsters, meaning that his home is filled with monsters that committed some crime (that for humans are just childish misdemeanours) and his cat is actually their jailor. Subverted when it turns out that he himself is the son of a human and a Tamer, a kind of monster that looks like humans and can control other monsters exiled due the crimes of some of them, thus he is the monster roommate, and Justified because his mother, who is actually a human, is a Keeper, a human who can see monsters and has volunteered to host a Oasis in her home.
- In the Bleach fic The World In Black And White, Ichigo, who became a Hollow after being killed by Grand Fisher, is roommates with Uryu. On the one hand, he's an oversized saurian Monstrous Humanoid who often slips into the Spirit World of Hueco Mundo for Soul Eating binges and whose spiritual energy attracts unwanted attention from Shinigami; on the other, he's Uryu's best friend and a dab hand at making tempura.
- My Little Pony: Friendship is Magic fics:
- This is the Life: A Tale of a Human in Equestria puts the title character and Octavia as roommates for some undisclosed reason.
- Another good example stemming from the same franchise is Bucking Nonsense's fic Five Changelings in Equestria: The Monster Under the Bed, where the changeling Ladybird evicts a boogeyman epyx for a young foal in exchange for under-the-bed and board priveleges.
- From Gensokyo 20XXV, we get this twice. The first one is more or less played straight with Mokou, Kaguya, and Suika, two immortals and an oni, and the second instance is played straight with Kosuzu (a human), who lives with a hermit (Miko), a celestial (Tenshi), and a tsurube-otoshi (Kisume).
- Soma and his roommates Naoki and Kazuya all pretend to be normal humans in Not the intended use, but turn out to be each other's monster roommate.
- At the end of Beetlejuice, the human Deetz family is living with the ghostly Maitlands.
- Han Solo from the Star Wars movies shares the Millennium Falcon with his Non-Human Sidekick, a seven-and-a-half-foot Wookiee named Chewbacca.
- Spaceballs riffs on the aforementioned Chewbacca with Barf, a Mog (half man, half dog — "I'm my own best friend!") who lives with the protagonist in a Winnebago.
- The short story Living with the Harpy by Tim Pratt is about, well, just guess.
- In Godshome by Robert Sheckley, it's technically a god roommate, but these gods are near enough to being monsters for most people. When Arthur Fenn gets his prayers answered, he doesn't realize that in payment, he'll get four very unpleasant gods living in his spare bedroom. They like their meat raw, and in large quantities, and they have a habit of throwing the gnawed bones out the window to rot in the yard, which eventually attracts the attention of the police.
- Monsters In My Mailbox: Reginald has several monster living somewhere in his room.
- The Addams Family (1964-1966) lives with The Thing, as well as frequent visitor Cousin Itt, and YMMV on the monstrousness of some of the others living in the house (eg. Lurch).
- The Munsters (1964-1966) inverted the trope (perhaps before it was fully developed) with a family of token monsters and human live-in relative Marilyn.
- On Mork & Mindy (1978-1982), Mork (played by Robin Williams) is a Human Alien and Mindy is his roommate.
- Buffy the Vampire Slayer. When she enters college, Buffy finds herself with the roommate from hell who has numerous annoying habits. Naturally this turns out to be literal with the roommate a soul-sucking demon.
- On Angel, Cordelia shares her apartment with "Phantom Dennis", the ghost of a previous tenant whose mother walled him up to prevent him from running off with his girlfriend. (Except we only rarely "saw" Dennis past the episode establishing him.)
- He was referenced or spoken to/of in anytime an episode featured Cordelia's apartment. He was even shown helping Cordy cover up the escalating damage her visions had been doing.
- Lorne also filled this role quite a bit in later seasons. See also the page quote. Both Buffy and Angel were regular employers of this trope.
- SARAH, the Smart House on Eureka, might qualify. Some people are certainly freaked by the idea of living inside HER and her actions/over-reactions in one of the later first season episodes made her the Monster of the Week when she thought Sheriff Carter was going to leave town.
- The show Being Human (UK), where a vampire and werewolf move into an apartment that they later realize is haunted by a ghost. They all get along better than you might expect. Similarly, its US remake.
- Harry And The Hendersons, about a family sheltering a friendly Bigfoot.
- Star Trek: Deep Space Nine: In "The Search, Part I", Odo briefly serves as this to Quark, literally and figuratively. Limited space on board the Defiant means that the two must share quarters, which becomes very uncomfortable for both men when Odo has to liquefy in order to rest.
Odo: I have been holding this shape for sixteen hours. I have to revert back to my liquid state, but I don't want you to watch and gawk at me.
Quark: I understand, completely. This is a very private moment and I won't interfere. This won't be so bad, sharing—
Odo: I HAVE NO INTEREST IN SPEAKING TO YOU, OR IN LISTENING TO YOUR WITLESS PRATTLE. SO STAY OUT OF MY WAY, OR YOU'LL REGRET THE DAY YOU EVER MET ME.
- The Soldier of Team Fortress 2 is roommates with a magician named Merasmus. They don't exactly get along well. Last Halloween Merasmus sent out his anger towards the entire team simply because he couldn't kill his roommate. Kinda fuzzy who's more monstrous though. Merasmus' last roommate? Tom Jones.
- Final Fantasy Legend II. In addition to being able to set your class as a monster child raised by human parents, other monsters are generally living with humans. They can be roommates, classmates, and/or teammates.
- In Touhou, this is why Reimu Hakurei doesn't get many donations. As the Hakurei Shrine's miko it's her job to deal with any youkai that cause trouble in Gensokyo, but nearly every one she battles succumbs to Defeat Means Friendship, leading them to drop by her shrine to hang out. Though Reimu herself doesn't mind - she's equally rude to humans, youkai and even deities - this does mean ordinary humans rarely make the trip to the Hakurei Shrine for fear of bumping into that drunk little oni who can tear apart mountains with her bare hands, or that "youkai of boundaries" who can do whatever the hell she wants.
- Randal's Monday: Mortimer is a mystery, but what does become clear after awhile is that he's done some pretty questionable things.
- David and Nancy note in Ow, my sanity.
- In a side-comic of Too Much Information, Maddie shares her room with what is, essentially, the larval form of a 'cosmic horror' type entity. He seems to exist mostly for the benefit of Fish out of Water jokes.
- A variation in 8-Bit Theater, where it's heavily implied that Fighter and Black Mage have been traveling and living together for a very long time (they met up after they both got kicked out of their respective schools (Black Mage for giving a new meaning to the word Evil, and Fighter for having the attention span of a rock) and they've been together ever since). It may have contributed to Black Mage's frequent bouts of omnicidal mania. Exhibit A.
- Tedd of El Goonish Shive lives with Grace, a shapeshifter whose ancestry is part-human, part-alien, part-squirrel, and part-second-alien. She was created to be a Super Soldier, but fled her Dark and Troubled Past and came to Tedd, hoping the technology he was experimenting with could help her defeat a powerful monster. It does. Also, they end up falling in love, and she moves in with him long-term. Which is more than a little awkward, since they're both under 18 and Tedd lives with his father.
- In Wapsi Square, Tepoztecal, the Aztec god of alcohol, has spent much of the comic living at Monica's place.
- Zombie Roomie: John is a human dork and George is a jerk and a zombie.
- Jareth for the other main cast members of Roommates. He's one of The Fair Folk (or at least pretty close, as his father is pure blooded fae noble and his mother a powerful witch) so generally not good for soul and sanity.
- Aylee from Sluggy Freelance is a mutating-shapeshifting xenomorph-type alien from another dimension, who is actually one of the more stable, reliable and friendly roommates in the house when her mutations aren't causing problems. Sam is a vampire who occasionally crashes with the gang. Depending on one's definition of "monster" Bun-bun might count as well, being a talking, misanthropic lop-eared rabbit who is one of the deadliest beings in the universe and possibly an amnesiac god.
- This Bigger Than Cheeses strip. Oh Xu'alz'kthar, you adorable scamp.
- Dielle in Rhapsodies lives in an extremely posh apartment belonging to the psychopomp Deidre. Apparently in exchange for taking care of Deidre's extremely demanding cat.
- In The Crawling City, Aria Wintermint is an ordinary young woman but lives with Gug, a huge tentacle monster. No explanation for this is given. Gug seems like an annoying but harmless roommate at first, mooching off her food and bugging her to buy him video games, but as we see more of this sinister city in which they live, it becomes clear that Gug has been protecting Aria, even going so far as to kill and devour a thief who broke into their apartment while Aria was asleep. Aria seem oblivious to this dark reality.
- The Cult of Scratchwood is a fan webseries which sees ordinary young man Matt forced to share his flat with a bunch of Daleks. (They are, for Daleks, unusually incompetent, fortunate for him.)
- CollegeHumor: The sketch "The Six Monsters You'll Have as Roommates" humorously characterizes the types of college roommates as various kinds of monsters: the antisocial control freak is a robot, the one you never see because he's always out is a ghost, the Guyliner-wearing goth who somehow always gets laid is a vampire, the foreign exchange student with strange customs is an alien, and the one who's always tired and lifeless is a zombie. The last Monster is the fact that there are no more monsters — i.e., you live alone, and are now completely freaked out at night because there's no-one to blame the noises in the night on.
- Lilo and Nani from all the Lilo & Stitch films and The Series have three aliens (Stitch, Jumba and Pleakley) living in their Hawaiian home, the first of them having originally been created to be a monster.
- In Ugly Americans, Mark lives with Randall, a zombie. This is a series where the monsters considerably outnumber the humans, so Mark doesn't see anything particularly strange about his living arrangements, even though Randall has brain cravings that seem reminiscent of a recovering alcoholic. (And he has the annoying habit of borrowing Mark's things without asking.)
- The Tick once faced against an evil alien named Thrakkorzog, ruler of dimensio... Errr... Apartment 14-B. He was roommates with a man who was barely even aware that he was sharing rent with a Blob Monster from outer space.
Arthur: Are you aware your roommate is a hideous monster from another dimension with evil plans for world domination?
Thrakkorzog's roommate: Listen, a good roommate relationship is based on a respect for privacy.
- Futurama: Human Fry lives with Bender, who, for those not familiar with the show, is a robot. Specifically, Fry lives in Bender's apartment. Fry is Bender's Monster Roommate. More than that: he lives in Bender's closet (which is as large as a 1 1/2 apartment), while the apartment itself is as large as, well, a closet.
- None of the Justice Friends (titular stars of the segment and show-within-a-show on Dexter's Laboratory) are exactly what you'd call normal, but The Infraggible Kronk is the most monstrous of the three, being a giant purple Expy of The Incredible Hulk.
- After being ejected from her home by a Corrupt Corporate Executive, human child Sari Sumdac spends the second season of Transformers Animated living with the Autobot protagonists. Then it turns out she's not actually human. She's a protoform who scanned her "father" and acquired a human alt-mode in the process. She's a Transformer just like her friends.
- An episode of Space Ghost Coast to Coast has Zorak rooming with Space Ghost briefly, much to his displeasure.
Space Ghost: ZORAK! Living with you is like living in a living nightmare!
- Die Sendung mit der Maus has "Trude's Flatmate". Selfsame mate is a completely nice and harmless monster...if it only had a whiff of knowledge of the local customs. Like you don't simply "borrow" a dredge for playing. (Think of Starfire, just less sexy.)