Klein is one of the bigger ones. He's introduced in the first episode as a new player who Kirito somewhat befriends. The two separate by the end but instead of making use of Klein by having him possibly act as a Foil to Kirito (mainly because Kirito fails to save the first guild he joined, Klein protected his whole guild through SAO), and him being an active ally in the series, instead Klein serves mainly as a comic relief character with not much characterization beyond being "Kirito's buddy".
If there's anyone who gets this worse than even Klein, it's his Guild, Fuurinkazan. Like their Leader Klein, they're frontline players who could stand toe-to-toe with larger guilds like the KoB and Divine Dragon Alliance, (the latter having been around since Floor 2), they have unique designs and weapons compared to the usual stock of swords the series favours, and they're stated to be the only frontline guild to have not suffered a single casualty throughout SAO. Despite this, unlike Klein who manages to just barely squeeze himself into every arc and storyline, they only appear in one arcnote two if you include Ordinal Scale, where they and Klein get a brief moment in the limelight. Even worse, it's stated that Klein plays ALO with them, but but we never actually see them play together in favour of Klein just hanging out with Kirito and his group. All in all, the most we learn about them is their respective weapon of choice and the fact they're roughly similar to Klein personality-wise. The only one of them to receive any focus is Dale, and that's just in Ordinal Scale as part of a Not-Death in the Limelight at the hands of Eiji.
Argo is one of the biggest examples. Thanks to Progressive turning her into the series' biggest Ascended Extra, she's much more prominent in the light novels as an information broker who Kirito frequents, and is known for to love endlessly trolling Kirito, but because the anime predated Progressive, it had her Demoted to Extra and only appearing in one episode for a couple minutes for a minor scene. Fortunately the manga and non-canon games give her a lot more time to shine and uses her potential where the anime failed to do so. However, her reappearance at the end of volume 21 and her status as a New Transfer Student implies that she's finally being reintegrated into the main cast properly after years of What Happened to the Mouse? since Aincrad, so now only time will show if it was worth bringing her back.
And then the Progressive movie came along and cut all of Argo's scenes from the original Light Novel, reducing her to a five second cameo at the very end.
Asuna falls into this after the Aincrad arc. While initially a strong female lead with a lot of focus and her own character, she gets reduced to a Badass in Distress in the Fairy Dance arc, while the subsequent Phantom Bullet arc has her staying away from the main action. Though this does get largely amended with the Mother's Rosario arc which has Asuna taking over as The Protagonist which fleshes out her character and gives her a lot of development. Something cemented further by her prominence in the movie Ordinal Scale, where she's the Deuteragonist once again to the same extent she was in the Aincrad arc.
Akihiko Kayaba, or rather the fact he was given no clear motivation for trapping thousands of players in SAO. When he's point-blank asked about it, he doesn't go into detail besides him basically saying "I did it cause I could". The fact that, before going into his rant, he just says "I forgot the reason a long time ago" doesn't help, since a lot of people take that line at face value, and consider Kawahara just didn't care about giving him a motivation. However, the subtext and the voice actor's delivery in the rant that comes after that line heavily imply that there could be other reasons. Likely him having some sort of God complex; not in the sense of having absolute power, but rather in the sense of being the creator of a living world out of his imagination, and he was obsessed with having his vision made (virtual) reality. Unfortunately, the series never explores this.
Many fans felt that Kyouji Shinkawa's transition from Best Friend into EntitledStalker with a Crush was this for him. Before The Reveal, many fans liked Kyouji for effectively being the Recon for Sinon's Leafa, enjoying their friendship and being glad that Shino seemingly has an alternative Love Interest instead of being regulated to Kirito's Unwanted Harem like most other girls. With The Reveal and, as if to add insult to injury, having him try and murder and rape Shino, many fans began to revile him, partially due to his wasted potential and wish for him to make a Heel–Face Turn in the future so he can be Rescued from the Scrappy Heap.
Yuuki Konno has the reflexes and skill to be Kirito's equal or even better, is lauded in-universe as the "Absolute Sword", and even Kirito states upfront that if she had taken part in the death game, she'd be the one who would have been given his Dual Wielding skill that he got... and she dies after one arc. To make matters worse, the very next arc introduces a perfected Brain Uploading technology that could've saved her.note Something that had already been foreshadowed, since Kayaba survived his "death" at the hands of Kirito and Asuna via a much riskier Brain Uploading. For obvious reasons, people prefer the videogame adaptations, where she DOESN'T die after her storyline, and instead is given the chance to shine as her fans feel she was meant to.
A non-character specific example would be in regards to the weapons. In general, there are very little characters who wield anything other than a sword, and those that do tend to receive very little focus. In the main cast alone, there's only fournote Sinon, Agil, Lisbeth, and Silica out of eleven combatant characters who don't wield a sword, and among the sword users there's only two that wield anything other than a longswordnote Asuna and Klein. Even a lot of the antagonists end up being sword wielders, and those that don't, such as Rosalia, are either dealt with quickly or receive little focus themselves.
Ordinal Scale
Sinon is marketed as a prominent character for the film, but ended up having very little involvement save for one battle.
Suguha is also underutilized in the film. She even has official art where she is wearing her Ordinal Scale outfit, but is never seen wearing it in the film itself, let alone participating in Ordinal Scale battles. She only appears for a brief scene in which she is departing for a Kendo tournament, thus she's gone for most of the film, until she makes a Big Damn Heroes entrance for the climatic final fight, but only as Leafa. Though there's some justification that as a kendo champion, she'd be a case of a Story-Breaker Power in a real life AR game as someone who is actually highly athletic, and this is at least shown in one of the Memory Defrag stories.
Alicization
Many people complain that Reki Kawahara never used Eugeo to the fullest of his potential. When we first meet him, we realize that Eugeo has more innate skill than Kirito, the beater and hero of Aincrad, and that he has a crush on Alice Zuberg, who was kidnapped by the knights of the Axiom Church and brainwashed, taking away her soul, memories, and old feelings, which is his motivation for going to Centralia and trying to become a knight. In the end, while Eugeo does play a huge role in Kirito's development even after the arc, Kawahara just kills him off. Kawahara even acknowledges that he made this blunder, mentioning that he was considering having the anime diverge from the Light Novel's canon by having Eugeo survive the fight with Quinella.
Alice Zuberg is another source for such complaints. When we meet her in volume 9 as a child, she is one of the most vivacious characters to ever bless the franchise and seems to have somewhat of a crush on Eugeo as a kid. Flash forward to her teenage years after her memories have been taken away from her and her old personality trapped in a golden cube— we meet her as a knight skilled beyond even Kirito and Eugeo in not just magic (which she was a prodigy in even as a kid), but she can easily match the two at the same time. She had a great deal of potential that was wasted in making her just another love interest for Kirito.
Ronye. While she and Tiese were introduced together as the valets to Kirito and Eugeo respectively, it is Tiese who gets virtually all of the focus and significance in Eugeo's Character Development, while Ronye largely remains unseen and her relationship with Kirito barely touched upon, despite how important a part she should have played in Kirito's life as his valet. Some go as far as to say the only part she played on the plot was to be victim to Attempted Rape at the hands of Humbert.War of Underworld does alleviate this to a degree by giving Ronye more focus, while Moon Cradle makes her the main character.
