Adorkable: Arthur is dorkily cute, easily frightened and somewhat high-strung, but you can't help but want the poor guy to pull through.
Alternative Character Interpretation: Due to subtle symbolism of the "Ghost" video, the characters as well as the past events leading to Lewis' death can be interpreted in multiple different ways.
In "Ghost", was Arthur merely a victim of possession when he was possessed by ??? and killed Lewis, or did jealousy for Lewis or fear of the cave make him more easy to possess?
Did/does Arthur love Vivi, making him jealous of Lewis, did he love Lewis and was jealous of Vivi, was he just jealous of their general dynamic and felt ignored, or was he even jealous of them at all?
Did Arthur lose his memory of Lewis's death due to the physical and mental trauma of having his arm ripped off and watching himself murder a friend, is his apparent memory loss a side effect of being possessed, did Vivi erase it subconsciously like she seems to be doing with photos of Lewis, or did Mystery tamper with his memory?
Mystery's motives. Where does he fit in the general scheme of things? Why is he masquerading as a dog? Why is Shiromori chasing him? There are many more questions than answers about him and his origins and many theories about all of them.
Did ??? posses Arthur to merely kill Lewis and who knows what else, or did it decide to kill Lewis due to finding jealousy or resentment for Lewis in Arthur after possessing him?
Aluminum Christmas Trees: Uncle Lance's double-barreled pump-action shotgun? It's based on a real thing, although it's not completely accurate, as it lacks the two triggers and 2 loading ports on the under side.
Awesome Music: The "Ghost" song itself is super catchy and shows that the popularity not only comes from the video or the characters, but the awesome music as well!
Crack Ship: Due to an artwork by one of the crew, depicting Arthur nervously blushing as Shiromori lays her arm on him with bedroom eyes, some fans have started to ship Arthur and Shiromori together.
Diagnosed by the Audience: Arthur is commonly headcanoned as having depression and PTSD following the cave incident due to his fear of Mystery (who ripped off his arm to save him) and complete neglect of his own self care during his search for Lewis, going days without eating or sleeping at a time according to Losing My Mind.
The Deadbeats. They don't affect the plot in any significant way, outside of acting like Lewis' henchmen and providing Foreshadowing Lewis' doubts in "Hellbent", but their Creepy Cute appearance caused them to become the franchise's Mascot Mook.
Out of Lewis' sisters, Cayenne seems to be the most popular.
Mushi has been quite well-received by fans, since many find the lore surrounding her fascinating, even though she's currently just a background detail for Shiromori's and Mystery's backstories.
Epileptic Trees: A minor prevalent theory is that Arthur was in love with Lewis instead of Vivi, mostly pertaining to the idea that there was no set-in-stone evidence that Arthur was jealous of Lewis specifically, or that the spirit possessing him exploited his envy in a different way.
What happened between Lewis and Arthur in "The Future" following Lewis breaking down and before Shiromori crashed into the truck is the subject of quite a bit of fanart and fanfic, usually of the Hurt/Comfort variety.
Thanks to the Mystery Skulls being a ghostbusting mystery-solving gang, it is very easy to imagine the kind of adventures that they would have off-screen. Especially when it comes to crossover-fics.
Arthur is often seen as suffering severe depression and/or PTSD after killing Lewis. Some fans even have the beginnings of it already in place beforehand due to him having been disowned by his parents for whatever reason, though it often involved him finally giving up on trying to meet their impossible standards, often with his uncle playing a major role in how he managed to pull away. Losing My Mind does at least show he has leftover memories of Mystery ripping off his arm, and he's terrified whenever he sees the inside of Mystery's mouth.
Before "Freaking Out", fan works frequently had Arthur know of Mystery's true nature while Vivi was unaware. Later videos implied neither knew outside of Arthur having a general unease around Mystery following the cave.
Lewis is often explicitly described as Latino in fan-works; canon itself is rather ambiguous about it.
It's common for Arthur to know one or both of the team since they were kids, usually Lewis.
Whether it's romantic or platonic, Vivi is very protective of both Lewis and Arthur and both of them agree that a furious Vivi is the scariest thing they can imagine.
If Lewis manages to come back into Arthur and Vivi's lives, he spends a while struggling to forgive Arthur for killing him, even if he's aware Arthur was possessed at the time, possibly even using his ghost powers to take petty vengeance on Arthur. Typically, this only really ends when either Vivi catches him and is furious, Arthur's life ends up on the line, or both.
Arthur as an electricity ghost is a staple in fanworks where he already is or becomes a ghost, owned to his yellow color scheme, passion for mechanics, and to complete the Fire, Ice, Lightning triangle set up by Lewis and Vivi.
For works involving post-cave ghost Arthur, it's common to depict his missing arm carrying over after death, with its "replacement" being made of energy instead.
It's common for fan writers to have the gang refer to each other by name shortenings; "Art" for Arthur, "Vi" for Vivi, and "Lew" for Lewis - Arthur and Lewis also have "Artie" and "Lewlew" as Affectionate Nicknames, with "Artie" seeing much more use.
In works where the Mystery Skulls sans Mystery don't know Shiromori's name, they usually just refer to her as a "tree lady".
It’s also pretty common to see Mystery referred to as a/the “(not)dog”.
Genius Bonus: Knowing something about the actual lore of the kitsune yokai gives some clue as to Mystery's motivations, though it leave other parts of his character an even bigger... well, mystery.note No kitsune of lore would be caught dead in a bob-tailed dog's body. In particular, Mystery has elements of both the inari kitsune - ancient, powerful and benevolent guardian spirits in the Shinto religion - and the yako kitsune - just as powerful, wild, and often implacably hostile toward humans. In his current state, Mystery seems to have appointed himself the inari-kami role to Vivi and the Mystery Skulls.
Ho Yay: Following "Hellbent", many fans have recontextualized Arthur's feelings for Lewis to be romantic rather than platonic.
Two instances in the flashback of "Ghost". In the beginning of the flashback, Arthur is shown looking sadly (possibly longingly) at Lewis, before looking away dejectedly, in direct contrast to his friends who are all smiling. Later, as they're walking into the cave, Arthur is performing a Security Cling to Lewis, showing that the other is a source of comfort for him on at least some level.
The general assumption is that '???' was able to posses Arthur due to his jealousy over Vivi and Lewis' relationship. However, who he's jealous of is left ambiguous at best. '???' forcing him to push Lewis could be interpreted as either Murder the Hypotenuse (if he's in love with Vivi), or a case of If I Can't Have You… (if he's in love with Lewis).
In "Freaking Out", Arthur, who has amnesia regarding Lewis' death, is shown with a map on his laptop that indicates he's been searching for Lewis since his "disappearance", to the point of exhaustion. In this scene, just as the magic effecting Vivi's memories is editing Lewis out of Arthur's screensaver, the lyrics "-but I'm afraid, afraid of losing you," plays.
The beginning of "Hellbent" is set to the song "Every Note"; the song's lyrics can either be interpreted as a case of Unrequited Love or as someone looking back on a relationship that ended badly. It also has the lyric, "Did you know when I told you that I loved you, baby?" This segment of the episode focuses heavily on Arthur, who has been searching for Lewis.
"Hellbent" also features a few interactions between Lewis and Arthur that put their feelings for each other into question. During the series of Flashback Cut that beats in and out of the background, two of Lewis' shows Arthur's depressed state over Lewis and Vivi's relationship. However, Arthur is very clearly looking back at where Lewis is during one of these, rather than at Vivi.
Idiosyncratic Ship Naming: The ship name for all three of the human (formerly human, in Lewis' case) members of the Mystery Skulls is referred to as "Poly Skulls", though the Portmanteau Couple Name "Lewvithur" is just as if not more common.
Many fans correctly guessed that Vivi has amnesia.
Prior to the official announcement, many fans predicted the third video would be set to "Hellbent" due to the credits for "Freaking Out" sampling from it.
The Losing My Mind prequel comic confirmed a ton of theories in one fell swoop:
Arthur's surname is indeed the furiously popular "Kingsmen", but it's spelled with an e rather than an a.
The pink aura emanating from Vivi's eyes was often theorized to make her forget about Lewis, or at least suppress her memories of him. The comic shows that when it pops up, she immediately turns her attention to something else.
It Was His Sled: With how viral "Ghost" immediately became, you'd be considered lucky going in not knowing that Lewis was a former member of the Mystery Skulls and was in a relationship with Vivi before being murdered by Arthur.
Jerkass Woobie: Lewis spends most of the series furiously gunning for Arthur's head, to the point of willingly endangering Vivi and Mystery over it... however, given that Lewis believes Arthur, a very close friend, betrayed and killed him simply so he could have Vivi to himself, it's not at all hard to see where he's coming from.
Lewis. Considering how he's a Tragic Villain, who still has a kind side hidden deep within, this shouldn't be a surprise. Many like to ship him with his girlfriend Vivi, but shipping him with Arthur or Shiromori isn't unheard of.
The frame of Lewis grabbing Arthur's shoulder is popular due to being a subtle Shout-Out to a popular Scooby-Doo meme.
A frame of Shiromori attacking the windshield of the van while Arthur and Mystery watch in terror is often used as a meme template; specifically, tagging Shiromori as something while Arthur and Mystery are tagged as something that conflicts it (e.g. "Me spamming my blog with MSA" vs "My followers", "Responsibilities" vs "Me trying to relax", etc.)
Vivi. She's beloved by the fanbase for being an AdorkableGenki Girl, and also a bit of a Woobie. Not too surprising, since she's an Expy of Velma Dinkley, who's considered to be quite adorable, too.
Mystery is also an adorable Funny Animal, though not so much in his original form.
Narm: In "Ghost," Arthur is shown running away from Lewis when it becomes clear that Lewis wants revenge. However, just before Arthur is shown panicking and running, there's a shot of Lewis's pompadour forming from a ball of fire; the way the scenes are framed, it can initially seem like Arthur is more scared of the pompadour than Lewis himself.
One True Threesome: The pairing of Arthur, Lewis, and Vivi seems to be more popular than any combination of just two of them.
Arthur has some feelings of jealousy or resentment towards Lewis and Vivi's relationship but doesn't vocalize it because they're so happy together. While exploring a cave, a spirit residing there takes advantage of Arthur's emotions and possesses him, using his body to push Lewis to his death to Arthur's absolute horror. The next thing Arthur knows, he's lost an arm and Lewis is missing, and with no memory of his death, Arthur works himself to exhaustion searching for him, only to get chased by a vengeful ghost that reveals himself to be Lewis moments before dropping him off a cliff.
Vivi was exploring the area under Lewis and Arthur and had a front row seat to her own boyfriend's death, visuals implying her breaking down before her memories were erased. Months later, she goes to sleep only to wake up to a crazy tree woman trying to kill her and her dog, and rushes to his side after Shiromori deals the killing blow, desperately trying to stop the bleeding before crying into his body.
For all he's a hysterically powerful yokai, you have to feel genuinely sorry for Mystery when he's forced to reveal his true form, which understandably terrifies Arthur. That's the moment when you realize that he cares about his human companions, and the idea of being alienated and alone, or allowing them to come to harm, really hurts him. If you know something about the lore of the kitsune it's even worse - whatever happened between Mushi and him, it cost him his tenko (divine) status - the point in which an inari kitsune gains their ninth tail and ascends to the heavens. Whatever his motivations, he's stuck among us - and has been for a very, very long time. "The Future" also reveals that despite whatever happened in their past, Mystery still cares deeply for Shiromori, and seeing her die is the final nail that allows ??? to take complete control of him.
While she was an antagonist, Shiromori herself is also quite sympathetic. Her origins were likely based on the Jubokko tree, a yokai from Japanese folklore. Based on this, as well as her actions and the flashbacks, it is likely that Mystery created her by feeding his blood to an ordinary tree hence her borderline addiction. Although he regrets it now after many centuries, in Shiromori's eyes Mystery turning his back on her was tantamount to abandonment and treating her existence as an Old Shame.
Visual Effects of Awesome: The series is smoothly animated with vibrant colors, quirky characters, and so much movement even the undead feel alive. The original "Ghost" in particular gives off a 70's vibe, fitting for its Scooby-Doo-styled parody, looking like it came straight out of Rhythm Heaven.
, although it's not completely accurate, as it lacks the two triggers and 2 loading ports on the under side.