In most long-running shows, it's normal for the writers to either lose interest in a character, or, at times, even completely forget about them. Since these characters are usually the easily forgettable ones, even viewers paid little attention to their sudden termination from the series and just assumed that they either died offscreen or just became victims of the Chuck Cunningham Syndrome.
However, there also times when some extremely promising characters come along, steal the show for a while, but unexpectedly fade away into nothingness. The world of anime characters, too, isn't bereft of these unacknowledged yet sometimes necessary character departures.
The Dark Bladers (Beyblade)
As Beyblade progressed from its opening season to season three, it inexplicably eradicated not just one, but many of its characters. Among these was a team of four bladers called The Dark Bladers (a.k.a Team WHO). These four characters were initially made out to be antagonists, known to be humans turned into soulless power-hungry monsters.
However, they eventually went from being Bit-Beast destroyers to reformed bladers after going through a major change during the Blademasters vs. Majestics battle. Soon after this, with their role in the overarching premise out of the way, they never made an appearance after season two and were never mentioned again.
Launch (Dragon Ball)
Considering how the Dragon Ball franchise has an ever-growing mythos and includes a roster of many characters, it isn't surprising that many of its seemingly pivotal characters have just disappeared over time. Launch, a dual-personality recurring villain, is among them.
After consistently appearing in Dragon Ball, Launch hardly made any appearance in Dragon Ball Z and was rarely talked about. She was last seen in the film Yo! Son Goku and His Friends Return!, and, as revealed by Dragon Ball creator Akira Toriyama, he had completely forgotten about her after Picolo's defeat.
Jigglypuff (Pokémon)
For the most part, Jigglypuff served as a running gag during the runtime of Pokémon. The cutesy creature appeared only to sing its sleep-inducing song and then get mad and draw on the faces of those who dozed off during its performance. However, after a few consistent appearances in the first series, the Pokémon was dropped and only returned once in Advanced Generation.
After its cameo in Advanced Generation, Jigglypuff appeared in the series again after over a decade only to disappear right after and never be seen.
Steel Saints (Saint Seiya)
The Steel Saints were anime original characters in Saint Seiya who were created by Dr. Asamori. What made them stand out among other Saints in the series was their ability to harness power from their armor instead of the cosmos.
While many fans of the series were initially intrigued by their unique abilities, they failed to leave a long-lasting mark in the overarching plot because of their lack of appearance after the Silver Saint Arc. Over the years, many fans have joked about their "vanishing act," claiming that they got on the wrong plane to Sanctuary, and, in turn, got separated from the Bronze Saints.
Usagi's Friends And Family (Sailor Moon)
Tsukino Usagi is one of the most prominent characters in the Sailor Moon franchise, but the same cannot be said for the supporting cast around her. Her parents and brother, who regularly appeared in the series, were written off a while after the inception of the R season. Later, the beginning of the Sailor Stars season marked their complete retirement from the series.
Other than her immediate family, even some of her closest friends including Naru and Umino appeared for the last time in the short Ami's First Love before their sudden exit from the series.
Ebisu (Naruto)
With Naruto's Chunin Exam match against Neji around the corner, Ebisu seemed like the highlight of the series for a while. His stern methods of training and absurd acts of perversion were enough to garner him some attention. However, soon after, Jiraiya came into the picture as Naruto's new trainer and completely stole the show, and there wasn't much more to be told about Ebisu's role in the series.
Even in Boruto, although many characters from Naruto have made at least some cameos, Ebisu remains to be in the Chuck Cunningham Syndrome abyss.
Doctor Shamal (Reborn!)
Independent assassin and former tutor of Hayato Gokudera, Doctor Shamal makes his debut in the series' ninth episode, and he sticks around to flaunt his deadly Trident mosquitoes. But, more than that, the Doctor always seems to be interested in wooing all of the women around him.
Strangely, though, right after getting a fleeting mention in the initial moments of the Future Arc, he is nowhere to be found. A Redditor came up with a hilarious theory claiming that he probably went to Italia after not being able to compete with future Tsuna's charm and abilities. Other theories put forward by the same Redditor suspect that he could either be dead after the fights with millefiore or just looking after Tsuna's family.
Nelliel Tu Odelschwanck (Bleach)
Everything about Nellie Tu Odelschwanck (a.k.a Nel)—from her physical demeanor to her personality—screams badass. That explains why, even to this day, many cosplayers aspire to look just like her. Clearly, her presence in itself was enough to snag the attention of most viewers, but she further cemented her place as a memorable character after her slambang showdown with Nnoitra.
To everyone's surprise, despite being so admirable, Nel was no longer featured in the series after the Kenpachi vs Nnoitra fight. Her absence sparked the rumors of her death, but her brief appearance in the novel Bleach: Letters from The Other Side confirmed that she's alive, though she was no longer a significant enough piece of the series' storyline.
Gentle Criminal And La Brava (My Hero Academia)
When dynamic villain duo Gentle Criminal and La Brava first appeared in the U.A Cultural Festival Arc, they seemed to have a significant role in the overarching plot. While Gentle's anti-hero traits suggested that he could potentially join the heroes later in the series, La Brava's quirk-enhancing abilities seemed promising enough to be useful for the heroes.
However, with the way things ended for the two in season four, it seems very less likely that they'll ever redeem themselves as heroes. It is pretty evident that they're both in prison by now for plotting an attack on U.A High School, but, beyond that, their existence will probably remain a mystery. Fans would still love to see them again, though.
Yamato (Naruto)
While most familiar faces from the original Naruto are still around in Boruto, Yamato has completely faded away. Naruto, Sasuke, and Sakura are still on the lookout for the new generation, and even Kakashi is out there mentoring Team 7.
However, it's quite unfortunate that Yamato's characterization did not find a place in Boruto's timeline even though he could still serve as a leading figure for Team 7, just like he did during Kakashi's absence in Naruto. Still, despite his lack of screentime in the new series, fans continue to remember him as a legendary ninja.