A character whose motivations and overall personality essentially revolve around their interaction with another, possibly more interesting, character with whom they really should be on an equal standing. Without this interaction, they would otherwise be pretty bland.
Sometimes a series will remove that "central" person from the equation somehow, and the Satellite Character has to establish their own motives. This is a common way to make a Battle Butler more interesting. On the other hand, a Spin-Off can fail entirely if the star is a Satellite Character whose character can't support their own stories.
Contributing to their poor image in some circles, badly-written Magical Girlfriends become Satellite Characters if not outright Satellite Love Interests with disturbing frequency.
Note that not all satellite characters are friendly, though... Some satellites are the exact opposite, full of all-consuming jealousy, rage, bitterness, vengefulness, or outright hatred, orbiting until the time is right to crash violently into the character they circle. This is EXCEEDINGLY rare, though.
Likely to count if the character is the Fat Girl, Black Best Friend, Pet Homosexual, or Uncool Redhead.
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Anime and Manga
Rin from InuYasha is largely defined by her interactions with Sesshomaru.
In Sailor Moon, the original author admitted Michiru was so unlike her she was difficult to write for, a trait carried over to the television show. The otherwise cryptic girl is largely defined by her interactions with Haruka, whom she usually appears alongside. In contrast, the fans liked Haruka's personality enough she got her own mini-arc and Back Story episode.
Mamoru was always like this in the anime. The writers ran out of anything to do with him once he hit the Relationship Ceiling, and he promptly became The Artifact.
Cromartie High School lampshades this with "You", the toady of the requisite snobby rich guy. Not even his boss knows his name, and the character has made numerous attempts to say it. Of course, all of them are interrupted by myriad events, from spilling juice to a meteor hitting the school.
Asuka in Yu-Gi-Oh! GX grew to serve less and less purpose over the course of the series outside her interest in the protagonist and The Rival's interest in her.
Not to mention Yubel, one of the aforementioned rare villainous examples. She was the Big Bad of the third season whose only defining trait is her obsessive love with Judai and the depraved lengths she is willing to go for the sake of that love. Take away Judai, and you have nothing left.
Michal, Lucia's love rival from Mermaid Melody Pichi Pichi Pitch, revolves almost completely around Kaito until she becomes Michel's mana battery. In fact, she does this intentionally, thinking that being completely devoted to Kaito in every way will help her steal him from Lucia before he gets his memory back, while her brother Rihito worries about her well-being for it.
Gopher from Soul Eater lives to please his master, Noah. He is, however, not very competent and very jealous.
Naruto, Shizune is this to Tsunade, downplayed in that it's more of an Overshadowed by Awesome thing; while she barely has a back story or much development, at the very least she's enjoyable to watch on her own.
Konan is one as well, as we don't know anything about her other than she uses paper as a weapon, likes to do origami, and that she followed Yahiko and Nagato around, fighting for the same goal as them. It pretty much becomes confirmed around chapter 449 when she calmly tells Naruto that since Nagato believed in him, she'll also believe in him. Without showing any emotion whatsoever towards the person who made her friend kill himself. She's more like a lost puppy trying to find a master than an actual person.
Nagato himself was one to Yahiko in the backstory. For all that he's supposedly prohpecied to change the world, he spends all his time orbiting Yahiko, and tells Jiraiya that all he really wants to do is protect Yahiko and Konan. After they form the Akatsuki, Nagato becomes Yahiko's Lancer and/or Dragon, dedicating himself to ensuring his friend's dream comes true. When Yahiko dies, and Nagato loses a planet around he snaps and becomes Pain...and is still trying to fullfill Yahiko's dream of a peaceful world, not so much because he cares, as because it was Yahiko's. Even after all this time, he's still chasing his friend's ghost.
Yamato was introduced into the series as being the substitute leader after Kakashi went out of commission. He was in the Anbu, was kidnapped and experimented on as an infant by Orochimaru, and is essentially the genetic clone of the most powerful shinobi who ever existed. On paper that sounds like one of the most interesting characters of the series. In actual practice? All Yamato has done in the series is help Kakashi train Naruto and play the straight man to Naruto and Killer B's antics.
And Chihiro has even less to call her own than Kaorin, being defined almost entirely as part of a pair with Kaorin herself. Kaorin at least is defined around Sakaki, Chihiro, and Kimura.
Sakura Kakei, the Team Mom of Get Backers, is largely defined by her relationship with Makubex, and rarely appears without him. And he's a minor supporting character himself (a Non-Action Guyunable to exist outside of Mugenjou).
No one knows what's up with Emiri Kimidori from Suzumiya Haruhi. The only things she ever does is have "silent conversations" with Yuki and play the nice girl for other people.
Also a Satellite Character is Kyon's Other Friend (Kunikida, accordign to The Other Wiki), at least in the anime series, who is mostly characterized as "That guy who hangs out with Kyon and Taniguchi" (to the point that in the novels he says he mostly hangs out with Taniguchi so he will have some in-universe characterization). Taniguchi is a lesser example, being Kyon's Only Sane Friend.
Rivalz Cardemonde from Code Geass is Lelouch's friend, member of the Ashford Student Council, and not much else beyond that. He doesn't really have a stake in the central conflict, relegating him to the status of background character, if that. Shirley, Milly and Kallen have more plot relevance than he does. Even Nina, who may as well have been invisible for the first eleven (no pun intended) episodes, has a bigger stake in the plot...though saying fan reaction to that has been "less than kind" would be a massive understatement.
Rivalz lampshades his status in the penultimate episode with Milly, lamenting that his friends are out there fighting for the fate of the world while he sits on his duff doing nothing but feeling sorry for himself. He actually is appreciated by some small measure of the fanbase for being quite attuned to his status as aSatellite Character. And several more fans find him cute or fun.
Even though she sets up plot-important gatherings and events at her school, Milly doesn't do much at all after she graduates from Ashford Academy.
A straighter example would be Kanon. We really don't know anything about him besides the fact that he's Schneizel's #2 (or something) and...that's really it. There's nothing else really notable about him and he never really does anything important.
In Chrono Crusade, Mary Magdalene's character mostly revolves around her interactions with Chrono. However, she's a creepier version than what's typical, particularly in the manga version (and in fact seems to have been meant to be slightly disturbing)—She's a seer that is so overwhelmed by the visions she has that she can't remember her own childhood or even her real name. There's only one piece of herself she clings onto—a dream she's had since she was a child of Chrono being the one to take her life. When she meets him, she willingly allows him to take her away from the safety of the religious order that guarded her because she wanted to get to know him before he killed her.
Sette of Magical Girl Lyrical Nanoha, whose entire personality revolved around her loyalty to her older sister Tre. This led to her becoming the only one of the younger Numbers to be imprisoned along with Jail and the older Numbers.
Matt in Death Note is an orphan at Wammy's house, but all we know about him apart from his loyalty to Mello is that he likes video games and cigarettes, and is gunned down while trying to get away from Takada's bodyguards.
Even the author admitted that Matt was the hardest character to write since he had "no idea what kind of a person he was."
The non-existentTsukihime anime did this to Akiha. She's Shiki's concerned sister, and... uh... Though this is somewhat remedied in the last few episodes, where she's still a borderline non-character, but is at least a borderline non-character with a backstory, an opinion on a couple of the other characters, and the illusion of being relevant to the plot.
In Fairy Tail, Droy and Jet are defined entirely by being Levi's bodyguards/fanboys and probably being very weak. Levi is far from important herself, so her attachments of course fail to garner any real characteristics of note themselves. Some of the less important minion characters like Shou are little different, but at least have their own personalities.
Juvia is an interesting subversion; she is madly in love with Gray, and he was her reason to join Fairy Tail, so for a start, one could easily think she would be Gray's Jet and Droy. And while her role in the story still is best summed up as "Gray's Stalker with a Crush", she gets lots of interesting interactions with other characters, always fights her own battles and in other ways has her own personality. Her crushing on Gray is mostly limited to comical scenes. So essentially, she has a role that one would almost always expect to be occupied by a Satellite Character, but she actually manages to not be a such character.
The fact that Riff from the Cain Saga (sort of a Battle Butler apart from rarely fighting) exists totally to serve his master becomes a plot point eventually. Especially when he turns out to be a brainwashed zombie planted by the lead's evil father to set up trauma when Alexis pushes the switch and Cain is betrayed by the person he trusts most. Of course, their relationship is explored fairly extensively, making Riff a possible subversion.
D.N.Angel has that one guy who is apparently a friend of Daisuke and Saehara, meaning Daisuke will sometimes ignore Saehara's fits to ask him what's happened, instead. A lot of googling will finally reveal his name as Sekimoto Masahiro; finding it mentioned in the manga itself is extremely hard. But he is fairly omnipresent, despite only speaking now and then to bemoan Saehara's antics. There are also the two class representatives; the only thing known about them is their names and relationship - the guy has a crush on the girl, who is oblivious and thus unintentionally cruel to him.
Villain example: Nappa from Dragon Ball Z exists mainly as The Brute to his boss Vegeta, having no real personality or characterization beyond being a mean bastard who likes smashing the Z-warriors into the ground.
Bleach: Tatsuki, Keigo and Mizuiro are defined by their role in the story as the classmates and friends of the story's main True Companions (Ichigo, Sado and Orihime and, later, Ishida). Tatsuki is additionally the friend of Orihime and Keigo's role expands a little to encompass Ikkaku and Yumichika. However, the interactions are still based on, and therefore revolve around, what is happening in Ichigo's life at that time.
Choujirou Sasakibe, lieutenant of the 1st division (the division of the captain-commander himself). He stays in the background of the story, barely receiving any prominance or dialogue and only ever heading into battle twice, being defeated both times. Justified in universe as it's revealed he made a vow to never become a captain, (he possesses bankai), never fight and to only serve Yamamoto rather than serving the Gotei 13 as a whole. In the end, the most important role he has in the story is to become the first protagonist to die in the Final Arc, thereby cementing both the power and attitude of the villains as well as putting the captain-commander on the backfoot immediately via the emotional fallout of Choujirou's death.
The various Digimon series occasionally fall into this with the partner Digimon - sometimes (Digimon Tamers) they're well-developed and have actual independent characterization and importance, whereas other times (Digimon Savers) they simply don't. Probably the most extreme example is MailBirdramon of Digimon Xros Wars - he has practically zero characterization and a handful of lines early in the series, and seems to exist solely to be a Mecha Expansion Pack for the much more fleshed-out Greymon, to the point that when they're merged into MetalGreymon/ZekeGreymon, said merger is almost always treated as just Greymon.
Digimon Frontier also brings this in with Kouichi, Kouji's twin brother, who becomes largely defined after his introduction to the series by his relationship with Kouji. Interestingly, the introduction of Kouichi also turns Kouji himself into this, as a large majority of Kouji's character-based scenes for the rest of the series likewise involve his interactions with Kouichi (although this can be overlooked somwehat because it was late in the series and Kouji had already gone through most of his Character Development).
Medaka Box: Despite being the male lead, much of Zenkichi Hitoyoshi's role gets boiled down to being the titular character's loyal childhood friend.
Homura Akemi in Puella Magi Madoka Magica. Her entire life revolves around Madoka. She has no past outside of being a Shrinking Violet who devoted her life to saving Madoka, and not a thing is said of her family or life prior to meeting Madoka.
Willow, B'loody Mary Smith, Ron and Ginny in My Immortal. Willow does little except be Ebony's best friend, B'loody Mary is slightly more developed but her sole purpose is to agree with Ebony's goffikness, and Ron and Ginny have no characters at all, except to be concerned with Ebony. It makes you wonder why the author went to the trouble to concoct new names and backgrounds for them, and then ditch them from the plot. The guys in Tom Satan Bombadil's gang seem to stand out of their own right, though their lives seem to revolve round their band and goffik stuff. To give Tara Gilesbie credit, this is true to life. Teenage girls tend to be satellite to the popular chick more than guys do, even when the popular chick (Ebony) is evidently being a bitch.
Gretchen from Mean Girls, toady in chief to Regina, who is possibly a Deconstruction in that she may actually be a more developed character than Regina herself; despite being beautiful and rich she is such an insecure mess that she is willing to put up with any amount of crap just to be follow Regina (seething bitterly all the while). At the end of the film it is revealed that she actually learns Cantonese just so she can join the new top clique.
Twilight brings us Bella, who despite being the protagonist, barely manages to stand on her own as character. Then when Edward shows up ... well, you know.
Ignoring Bella, the majority of her human admirers and friends have no characterization, back-story, or purpose outside of being Bella's long-suffering lackeys. Mike's about the only one with some depth, and even he is pushed to the darkest corners of the book when Edward shows up, only popping back out now and then to be jealous of Edward.
Rosalie, Jasper, Emmett, Victoria, Esme, and Laurent were all this in Twilight. Rosalie's sole purpose exists to be the bitchy blonde foil to Bella. Emmett and Jasper serve no purpose but to be husbands to Rosalie and Alice respectively, and Esme is only really there to be Carlisle's wife. Victoria and Laurent, meanwhile, are defined through their interactions with James - Laurent's only purpose is to warn the Cullens how powerful James is, and Victoria acts as James's back-up/mate.
Harry Potter: Vincent Crabbe and Gregory Goyle, for the first six books, are Draco Malfoy's two dumb cronies. And...that's it. Despite their frequent mentions throughout the series, neither one of them even gets a single line of dialogue until Deathly Hallows, when Crabbe finally gets some characterization — he's not very nice, and he dies within pages of his first line. This all makes sense, though; for the first six books, Crabbe and Goyle were intimidated by Draco (and his father) and were perfectly willing to be little more than Draco's cronies, but when the Malfoy family falls out of favor with the Ministry of Magic, the Death Eaters, and Voldemort himself, Crabbe and Goyle abandon their leader. Under the Carrows, Crabbe and Goyle unleash their sadistic natures. (Yeah, Goyle's still pretty much just Crabbe's sidekick, but at least one of them took the initiative.)
Also, Wormtail. He basically finds the most influential person he has access to and sucks up like nobody's business. He lived as someone's pet for years—literally—just so he could avoid braving the world on his own under the SLIGHT chance that someone might have discovered his incredibly well-concealed Death Eater crimes. One wonders how he got into Gryffindor in the first place.
Similar to Bella, Nora Grey from Hush Hush is this. The only reason at all she's involved in the plot of the first book is because of her interactions with Patch. Because Patch loves her, Jules and Dabria want to kill her. If he wasn't interested in her, they wouldn't look twice at her. Not even being in a relationship with Patch is her own doing, because they officially hook up after he declares that they should do so, giving her pretty much no say in the matter.
Hazel literally has one character-centric episode (ironically, it was in the first season, before her Promotion to Opening Titles). The episode has to do with her not wanting to embrace her Somali Muslim ethnicity, and is a pretty interesting and topical episode. Other than that, there are only a few other episodes that feature her prominently... interacting with Jimmy, Paige, and others in their far-more interesting problems.
And another more annoying example is Chantay Black, who somehow managed to be introduced in Season 4, do nothing for literally 4 seasons, be put in the title credits, and do a whole lot more of nothing before she begins dating Danny Van Zandt. The producers must feel really bad for the actress or something.
Then of course there's Leia, who does pretty much nothing other than just existing (Coincidentially she also dated Danny Van Zandt). The fact that she just kind of appeared out of nowhere and was instantly shown in the opening credits makes it even more noticable. Also, her being asian doesn't help...
In Season 7, Anya and Sav are in the story strictly to further drama between Mia and Holly J. While they have gotten better, they spend a lot more time as support to other plots then they do on their own plot.
Blue was introduced as a love interest for Holly J in season 8, they break up in season 9, he leaves the show before season 10. Given his only interaction outside of Holly J was one episode with Riley and he didn't do much in it. Safe to say, when he leaves, nothing of value was lost.
Zane's not a bad or flat character and actually became a bit of an Ensemble Dark Horse, it's just that all of his storylines revolved around Riley and when they broke up he disappeared until graduation.
This also happens to Jenna in the second half of Season 12. She became more prominent in Seasons 9-11, but her role generally started to fade at the beginning of Season 12. Her only episode focus she's had so far was in Closer to Free, where she befriends new girl Becky Baker and tries to find her faith in god and by doing so, she gets baptized. She was only doing it to try and get closer to her brother Luke and in part 2 of Rusty Cage (which wasn't exactly her A Day in the Limelight episode), when KC and his mom had to leave Degrassi to get away from his abusive father, he tells Jenna to say goodbye to their son Ty for him. Her plot only revolved around KC's plot for that matter. After that, she became pretty much of a background character for the most part.
In Boston Public, the school coach was billed as a main character in the opening credits, but his entire role seemed to be commenting on other people's plotlines. The Television Without Pity recaps dubbed him "Coach Lamphrey the Plot Parasite."
Although Vala is more than capable of standing on her own feet as a character in Stargate SG-1, she is noticeably much more interesting and funny when she's teasing Daniel. In fact, she often seems more lost and bored without him, and definitely much less manageable.
The families of Rose, Martha, Donna and Rory in Doctor Who become recurring satellite characters of one form or another for as long as they remained on the show (and not one second longer). Averted with Donna's grandfather Wilf, who becomes a companion in his own right, although briefly.
The classic series had all the minor UNIT people who were satellites to the Brigadier.
Then there's Rory, who starts off like this, then dies, gets erased from existence, comes back with his soul in a plastic Auton body, then properly revived and graduated to full companion status.
The Adventures Of Brisco County Jr had an episode featuring the notorious (and entirely fictional) Utah Johnny Montana, a master duelist and bounty hunter. He was unable to speak due to a bullet wound to the throat, so the man he kept with him at all times said everything for him, fully aware of his intent despite never receiving any cues. When Montana is defeated, he even collapses as though he was the one defeated.
David Harewood's Tuck from Robin Hood gets some interaction with Guy and the outlaws in his first episode, but from then on out, he barely interacts with anyone except Robin, acting as his P.R. spokesperson.
On LOST, there's Niki and Paulo. When alone, Paulo stood up better than Niki did. And in season four Charlotte is a Satellite Character who is best when interacting with Daniel.
Guinevere from Merlin has no real Character Arc of her own, at least not one that isn't connected to her relationships with Morgana, Arthur or Merlin. She does have a father and a brother, but the former was killed off and she barely interacts with the latter. That said, it's also an example of Tropes Are Not Bad, as her relationships with Merlin, Arthur and Morgana make up a significant part of the show.
Buffy the Vampire Slayer had Oz. Even in spite of him being a werewolf which seems like it would be a potentially interesting backstory it often times barely got glossed over, and felt like the only real time he got to shine was when he was with Willow. This is one of the reasons Seth Green eventually left the show.
Andrew Ridgeley of Wham! is one of, if not the most notable musical example. Although Wham! were always presented as a duo of George Michael and Andrew Ridgeley, George Michael was the one who wrote the songs and had the most star appeal. This was confirmed when George Michael's solo career became very successful, but Ridgeley's sank without trace.
Exaggerated with Kitty, the family pet in Polly and her Pals. 90 % of Kitty's actions can be summed up as, "follow Pa around and perfectly mirror his mannerisms and expressions". Naturally, this was Played for Laughs.
In Fox Trot, Nicole and Steve pretty much existed only to be the respective friends of Paige and Peter. Morton Goldthwait and Eileen Jacobson also spent most of their time in the strip as little more than antagonists for Paige and Jason, respectively, with little to no interaction with any other character. Morton is also Peter's classmate and, at one time, Jason's camp counselor, but neither of those roles is quite as prominent as his relationship with Paige.
Due to limits on how much story time (Developers usually have to spend a lot of time tweaking the gameplay, so if there's a minor character they're just there to take up space) there is to develop characters with, non-RPG video games have far too many examples of this to list, mostly in the form of rivals, love interests, parents, slain mentors and other already-cliched roles.
Angry variant: While Wario's main characteristic is his greed, Waluigi seems to be defined primarily by his hatred of Luigi. Later games have given him an egotistical cheater personality, but only once has he featured without Wario (Mario Tennis: Power Tour) and is never not seen competing with Luigi. He has never appeared in his own (official) game. He was created for Mario Tennis and has since only appeared in the multi-player party and sports games. He's had a cameo in the Warioware games and a loose reference in New Super Mario Bros. Wii, but that's about it.
Nintendo seems to be fond of introducing these characters in the Mario side games but unlike Waluigi, they tend to be from previous games (Daisy, Bowser Jr.) or rarely come back (the Koopa Kids).
In Final Fantasy XII, Penelo's official character description says it all: "Vaan's friend."
Fleshed out in succeeding Ivalice games, thankfully. It's sort of sad to have a Satellite Character being the satellite to, well... Vaan.
Luxord of Organization XIII's claim to fame is being able to insert the word "game" into any sentence. And that's it for characterization.
Demyx isn't much better, with his only distinctive character traits being ad-libbed by his original Japanese voice actor. However, due to his boss battle (for guys), and his looks, he has actually become an Ensemble Dark Horse of ridiculous proportions.
Or Larxene to Marluxia. Or Lexaus to Zexion. Or... most of the Organization is this, frankly. Luxord lampshades it by saying, "If only the whispers from the top of the ladder carried to the bottom rung."
Thanks to having 108 characters, Suikoden can be pretty bad about this. Suikoden Tierkreis in particular has satellites to satellites (for instance, Megion's male follower, or the two kids who argue about whether Fredegund should be forgiven).
Several characters in the Soul Series are this, especially as the series becomes more evidently about only a select few characters. These include Setsuka, the Unknown Rival to Mitsurugi, who already has a rival; Amy, the foster Daughter to Raphael who himself has already served his importance to the main plot; and several other characters who were either never important or have concluded their importance.
Quarr, an Imp who serves as a minion to various minor villains, gives us these sagelike words of advice: "The first thing you learn when you're my height is to find someone who's bigger to you and latch your lips firmly on their ass." Subverted in that he can come up with decent schemes on his own, and occasionally summon the odd demon who owes him a favor, but left completely to himself, he's capable of fighting V's bird to a draw and not much else.
Shep in Schlock Mercenary started out as one of Those Two Guys, but as Nick got more and more fleshed out, it started to seem like Shep's only use in the strip was as an occasional foil. Eventually, the author caught on and Shep was Put on a Bus.
Trike Girl in Sinfest doesn't really do anything except show up to do something to oppose The Patriarchy.
Most new characters in Sinfest get their start this way, including Lil' E, all of the devil women, coffee shop lady, and others. Apparently Character Development being included in the benefits package makes entry-level Satellite Character work attractive at the comic.
Another angry variant: in the Whateley Universe, Tempest is pretty much defined by her massive hatred of Chaka (and by extension, all of Chaka's friends).
Survival of the Fittest has Elizabeth Priestly, who plays the satellite to her twin brother, Lenny. Most of what she does consists of following Lenny around and angsting about his psychotic behavior.
This is spoofed with the character "Star" on Danny Phantom, a preppy blonde who Tucker nicknames "Satellite," because she is "the marginally attractive girl who always orbits around the popular girl". Before this, she'd received a few solo cameos, but ever since the joke, her appearances have almost always been beside Paulina, the popular girl in question.
Hartman got pretty bad at this. In early episodes of The Fairly Oddparents, some characters like Trixie and Chester would be hinted as having hidden depths. Less so in later series. Danny Phantom hammers home the "cliques" theme so hard that the characters literally go out of their way to define themselves according to their placement on the social hierarchy—the Popular Kids aren't "like a club," they are a club. The teacher can be summed up as "stealth mentor to Danny," "benefactor to the cool kids," and "Zoidberg" to everyone.
Angry variant: Depth Charge, from Transformers: Beast Wars, is an Ahab-esque satellite character defined by his all-consuming hatred for the monstrous Rampage. It's implied that Rampage set this up intentionally.
Also played straight on Beast Wars by Inferno, a Predacon pyromaniac with the mindset of a real ant, who considers Megatron to be "The Royalty".
Jealous variant: Veronica Star from The Fairly Odd Parents, is Trixie Tang's satellite. She is in love with Timmy, and wishes that she could be Trixie, because Timmy likes Trixie.
Disney sidekicks by their very nature, whether good or bad characters. Possibly the most glaring (and inexplicable) example is Lefou, Gaston's little stooge in Beauty and the Beast. Lefou spends most of his time running errands for his 'friend', only to be constantly abused and beaten up for his pains. And yet still he insists he thinks Gaston's wonderful. If Lefou was a girl the implications would be very disturbing.
Stacy and Tiffany from Daria were this in the first season, they essentially filled up the fashion club and served to either converse with Quinn or Sandy, the characters of real importance about matters related to their feelings or the plot. However, season two onwards saw more characterization with respects to Stacy, actually making her highly sympathetic. Tiffany, however, stayed flat.
Single Minded Friends Katie and Sadie from Total Drama are this for each other. Fanfics try to avert this (usually for shipping reasons), but the show's writers haven't given them a drop of development. Even when Katie's elimination forcibly separated them for five episodes, the best the writers seemed able to think of was demoting Sadie to a silent extra for most of them.
Tyler for Lindsay in the first season. Luckily he receives some development in World Tour.
Cody's only purpose in Island was to be Gwen's creepy Hopeless Suitor, after he does the I Want My Beloved to Be Happy shtick in Island, he gets eliminated. In a very painful way, then in World Tour his only purpose, along with the above, was to be the target of Sierra's affection (whose only purpose was to show how pathetic and nucking futs Cody's real-life Fangirls are)
High Five Ghost and Eileen of Regular Show seem only to exist to stand mostly silently next to Muscle Man and Margaret, though Eileen's been getting quite a bit of Character Development.
Depending on the episode in Goof Troop, occasionally PJ and Pistol will be little more than extra mouths to agree with Peg or Pete and will be treated as a single entity of "the kids". In most of these episodes, both the Sibling Yin-Yang relationship and Pete's Parental Favoritism apparent in the rest of the series are removed, though there are a few that still hint at them.
In Gravity Falls, the "Cute Biker" is hardly ever seen in any context besides cheering on Manly Dan.
The same goes with Manly Dan's three sons, all they ever do when seen is cheer on their dad when he does something manly and join in on their dad's activities... we don't even know their names!
Real Life example, Ant and Dec. As The Now Show once said, "You can have one without the other but what would be the point."
Every school probably has a variation of this; probably every workplace, too.