Adorkable: Randy and Howard can be quite dorky and adorable, but especially Randy.
Alternative Character Interpretation: NomiRandy. Is he the Nomicon's consciousness or a replica of Randy's subconsciousness? A fusion of the two is entirely possible.
"Aplopalypse Now" reveals that Plop Plop, the loyal friend and esquire of the First Ninja, was forced to become the guardian of one of the Sorcerer's balls for 800 years against his will. (By his best friend, no less) Despite that, when he meets Randy and Howard, he shows no signs of being remotely affected by this, and at the end of the episode When Randy uses the Nomicon in order to destank him, Plop Plop is overjoyed to be reunited with the First Ninja again.
You'd think Julian would be a bit less chipper after becoming trapped in The Land of Shadows by his own evil doppelganger, but when Randy and Howard come across him there in "Ball's Well That Friends Well", he isn't behaving much differently, and seems to have gotten used to his new home. Though, this could potentially be seen as some delirium caused by isolation, seeing as he's befriended a seemingly inanimate gargoyle skull.
Awesome Art: The series has a nice art style and design courtesy of Jhonen Vasquez, the animation doesn't fall behind either as it has a pretty unique style to it. With it's action scenes being dynamic and rather bombastic.
Base-Breaking Character: Howard Weinerman. He's either a funny character whom at times shows that he isn't completely useless and shows that he cares for Randy, or an insufferable Jerkass with Skewed Priorities whom is a toxic friend to Randy. Not helping with this is part of the fandom's tendency to excuse his worse moments as "just part of his character" while leaping on Randy for the slightest jerk-ish inclination (though this has lessened recently).
Cargo Ship: Randy and the Ninjanomicon, there's even a moment where he tries to make up with the nomicon and lovingly rubs his face against it. Leaving Howard a little creeped out and just giving the two some space. Like with most cargo ships, the fandom has created a Moe Anthropomorphism of the object half.
Julian, the goth boy, is a very popular character within the fandom. Mainly thanks to his adorable Perky Goth mannerisms. His popularity could be a contribution to him becoming an Ascended Extra in the form of his Evil Doppelganger, Shadow Julian. Which has also become a rather popular character in his very own wright.
Morgan, the leader of "Dancing Fish", is another very popular character.
The Tengu has a huge amount of fanart.
Theresa is ridiculously popular for the amount of screentime she's had, to the point where there are several Ninja Theresa AUs and other fan ideas with her as the protagonist floating around the fandom.
Rachel, the clarinet girl who is also a big fan of the Ninja.
Evil Is Cool: In-universe, Randy and Howard would love to boycott McFist's products but they're just too cool for them to pass up on purchasing them. Slips into Fridge Brilliance when one realizes they'd stick out if they stopped buying things from the most beloved man in town.
The Ninjanomicon, already having the canon nickname “Nomicon,” has been occasionally known as “Nomi” within the fandom, mostly associating with its Moe Anthropomorphic form (also courtesy of the fandom).
The series itself has been called Ninjashow, Ninja Show, and other variations. Mostly, curious as it seems, when anticipating the next episode (i.e. ‘I can’t wait for more Ninjashow,’ ‘I gotta have my weekly dose of Ninjashow,’ with the exception of ‘I’ve been catching up on some Ninjashow lately’). In general, the Tumblr fandom almost never uses the full name of the show; the main tag is #rc9gn, and users will either refer to it as that or Ninjashow - even the production blog is just called 9th Grade Ninja.
Evil Julian is often referred to as Ghoulian due to his white, ghostly color scheme.
The Ninja of '05 is lovingly referred to by the fandom as "Ninja Kronk", mainly thanks to being voiced by Patrick Warburton and having a very similar physique to him.
Randy and Theresa was this originally, but Theresa is now Randy's canon love interest.
Heidi and Randy is another very popular couple, as well.
Howard and Debbie are often paired together too.
Not to mention the most popular ship in the fandom, Randy and Howard.
Another famous ship is Randy and Nomicon.
Julian and Theresa.
Fandom Rivalry: Due to the similarities with the plot, character traits, and inspiration from Spider-Man, some members of the show's fandom seem to be bitter towards Miraculous Ladybug, especially because the show became more popular than Randy Cunningham: 9th Grade Ninja. These members also sometimes accuse the former show of plagiarism. Likewise, some Miraculous Ladybug fans aren't friendly with this show either, due to it's excessive crude humor.
Thanks to Spiritual Adaptation and a popular webcomic called Age Accurate Secret Trio, crossovers are common between this show, American Dragon: Jake Long, and Danny Phantom. Also thanks to the aforementioned webcomic, these kinds of stories are commonly tagged as "secret trio".
The fandom can be pretty friendly with the Miraculous Ladybug fandom, due to how similar the shows are. Sometimes, the Secret Trio fandom expands itself to allow Marinette or Adrien (or sometimes both as one identity) as a member. However, there is also a small portion of the Randy Cunningham: 9th Grade Ninja fandom and the Miraculous Ladybug fandom that have a rivalry against each other, for the same reason.
There's also an overlap between this show's fandom and the Detentionaire fandom.
Growing the Beard: While Season One was good, Season Two became Darker and Edgier with Story Arc, less Toilet Humor improved animation and more focus on Character Development. While Randy was given character development all throughout Season One, Season Two featured a handful of episodes centering on Howard's flaws, with much needed development for him.
Episodes Shloomp! There It Is and Randy Cunningham: 13th Century Ninja is when the series really picked up.
In the first episode, Viceroy says that the Ninja's suit could be navy blue. Years later, Ben Schwartz would be type-casted as blue-coloredcharacters.
On that note, in one episode, Randy and Howard sneak out of the latter's house to see a movie based on their favorite videogame. And one of the blue-related roles Ben Schwartz voiced years later? Sonic the Hedgehog.
And when the Viceroy's highly advanced projector fails (thanks to Randy and Howard sneaking in to see the movie), the film's main character comes to life and starts attacking the viewers, but Randy and Howard mistake this as part of the movie and the former claims its better than the game. While the Sonic movie isn't considered the best entry in the franchise, it did earned a lot more love from the fandom than the most recent Sonic gamesnote except for Sonic Mania..
Ho Yay: The relationship between Randy and Howard runs close to this a lot.
Season two ramps it up by having an episode where Randy gets jealous of Howard hanging out with Bucky (titled "True Bromance") and where they have to take care of a bologna "baby" for science class and bicker over who gets to look after it.
LGBT Fanbase: There's a decent amount of LGBT fans for the show, mainly thanks to Viceroy, whom is a well-loved villain for being an effeminete, sassy, fabulous Deadpan Snarker and Evil Genius, who also as a few Ho Yay moments between him and his boss, Hannibal McFist. There's also a good amount of Ho Yay situations involving Randy and Howard, whose friendship can go from either close Vitriolic Best Buds or bickering Like an Old Married Couple.
Moe: There's no sugar coating it-Randy is adorable.
Rachael may be this In-Universe, so of course some people in real would feel the same way.
Julian could possibly be the best example, being the perkiest Perky Goth ever and having a cute appearance to match. The soft voice (courtesy of Dee Bradley Baker) helps.
Moral Event Horizon: Shadow Julian crossed it when he controlled Howard's mind and had him walk towards a pit, giving Randy a Sadistic Choice to either stop him or save Howard.
In the Japanese dub, this was for Tomoaki Maeno as the titular Randy.
Ship-to-Ship Combat: Slowly has been starting between Randy/Theresa and Randy/Heidi shippers.
Not to mention the ship wars that occurred between Randy/Theresa and Randy/Howard fans when Randy/Theresa was confirmed.
Tear Jerker: Quite a few moments from the Evil Julian arc, especially where Julian himself is involved. It's sad to see such a sweet character suffer, especially since he's usually so happy, and he really tries to make the most of his bad situation.
They Changed It, Now It Sucks!: For fans of Tim Curry, the change of his voice to Ben Cross for season two would count as this. note Though a reminder that Tim suffered a stroke at that point and could no longer voice act like he used to. So the change was necessary.
The Sorceress. She only appeared for two episodes and hasn't even been mentioned since making her an Aborted Arc.
Theresa and Debbie due to being rather popular among fans and even getting some focus episodes, making them prime candidates to move into a Main Character slot, but they always end up pushed back into the background after the episode focus is done.
Heidi as well, as she is the girl who appears in the most episodes with a speaking line, but is usually just dumping info on the setting.
Like above, it's most of the female characters including Morgan and Rachel.
Despite playing a major part in the Evil Julian arc, some fans feel there wasn't enough focus on Julian. Specifically, he was stuck in a shadow dimension and there was a bit of filler during this arc, meaning that most of the focus still went to Randy and Howard despite Julian being a driving force.
To a lesser degree, Evil Julian himself feels underutilized, despite being one of the second season's main villains. His appearances are very spaced out across five of twenty-four episodes, and his plot is wrapped up rather quickly in the finale. Considering the writers intended for Evil Julian's story to last longer, the series' cancellation likely had something to do with the hasty wrap-up.
In general, many new villains outside of the Sorcerer, McFist and Shadow Julian (Mac Antfee, Catfish Booray, the Sorceress) only get two episodes about them before their plots are resolved, which is too soon in the eyes of many fans.
After Randy pretends he's sick to stay home from school in Randy Cunningham's Day Off, Howard calls McFist and tells him the ninja's not in school today. McFist takes the opportunity, but Viceroy points out the concern that someone out there knows he wants to destroy the ninja.
Viewer Gender Confusion: Julian, to some, due to his androgynous appearance and quiet voice. The fact that he's partially inspired by Noel Fielding-who also falls under this sometimes-doesn't help much.
a Moe Anthropomorphism of the object half.