Ensemble Dark Horse / Sword Art Online

Due to the number of characters present in Sword Art Online, it's unsurprising that several minor and unimportant characters became fan favorites.

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Debuting in Aincrad

• Argo is easily considered one of the most interesting characters and one of the biggest examples of wasted potential in the series. Her presence in the light novel gives her a very colorful personality of sarcasm and greed as a Knowledge Broker. She's also loved for endlessly trolling Kirito and Asuna. The anime had her Demoted to Extra and only appearing in a brief scene for one episode. Notably, the Progressive rewrites for the Aincrad Arc went out of their way to make Argo a recurring side character and Ascended Extra, and the whole game continuity turned her into a Guest-Star Party Member and potential love interest for Kirito before being Promoted to Playable as the games went along. This finally reached its apex with her surprise reappearance at the very end of volume 21 of the light novels, with the implication of reintegrating into the main cast.
• Lisbeth and Silica. While they don't really qualify these days anymore given their status as Breakout Characters who were made into Ascended Extras. These two girls were insanely popular back in the days of Season 1, despite only having a single episode each focusing on them. They ended up becoming two of the most heavily marketed characters of SAO, dominating merchandise and with prominent focus in video games. They eventually even gained their own spin-off in the form of "Girls Ops" where they, along with Leafa, serve as co-leads of the series with Kirito completely out of the picture.
• Klein and Agil are HUGE examples. As two of the most likable characters in the series, a large amount of fans wanted THEM to have their own spin-off and even Kawahara agreed with it. Klein takes the cake for acting like such a bro to Kirito, showing genuine emotion, and being a damn good fighter and guild leader whose entire guild survived from inception to finish. While Agil is the responsible Token Adult among the kids who acts as the voice of reason and even runs a Good-Guy Bar, with just a bit of Vitriolic Best Buds with Kirito thrown in.
• Sachi, Kirito's first love who tragically lost her life. Her single episode is often hailed as the best in the Aincrad arc and virtually everyone was saddened to see her go. It's not uncommon for people to wish Kirito ended up with her instead of Asuna, with some feeling his love for Sachi was more believable. Unlike Yuuki, fans were further disappointed that Sachi was not Spared by the Adaptation in the video games.
• Kizmel from Progressive, an NPC Dark Elf who displays a very advanced AI and befriends both Kirito and Asuna. Regarded as one of the most interesting characters from Aincrad with many wishing she had appeared in the anime, and the game continuity even allowed her to become a playable character in Hollow Realization after completing her side quest.
• The two players from the pilot who are revealed to be a G.I.R.L. and an overweight male, with the G.I.R.L. wearing the female armor in all his subsequent appearances. For a pair of unnamed characters with a few scattered cameos, they get a huge amount of attention in the fandom. Their possible Relationship Upgrade was pretty well received, as is their reunion in real life where they embrace upon physically meeting for the first time. Integral Factor finally provides their names with the G.I.R.L. named Miyurin and the fat guy named Ulrik.
• Godfrey, who is killed off after 4 minutes of screentime is this. His bro-ness personality and appearance reminds viewers of another Ensemble Dark Horse: Rider.
• The Gleam Eyes is fondly remembered as one of, if not the most, iconic bosses of Sword Art Online. It was one of the most brutal boss fights shown, and forced Kirito to unleash his Dual Wielding unique skill to put an end to its threat, resulting in a truly spectacular visual display.
• A small niche of fans (and some non-fans, ChubbyChasers, and furries/scalies) tend to like Illfang the Kobold Lord due to his design.

Debuting in Alfheim Online

• Alicia Rue the Cait Sith Lord. A very perky girl who cheekily flirts with Kirito. Quite memorable as a character despite her limited scenes.
• General Eugene, the Commander of the Salamanders. His duel with Kirito is regarded as one of the most memorable moments of the divisive Fairy Dance arc and he is notably one of the few cheats characters who is neither a main cast nor a villain to become a Worthy Opponent to Kirito. Made even better is the fact that he has a Defeat Means Friendship moment with Kirito meaning he can return without trouble.
• Kuro/Lux from the Girls Ops spin-off manga, who is a Distaff Counterpart and Identical Stranger to Kirito. However she's much more warmly received than Kirito for her more complex personality and sympathetic backstory.
• Tonkii the elephant-jellyfish hybrid from the Calibur arc that Leafa befriends is quite loved among the fandom.

Debuting in Gun Gale Online

• While not really a true character, Kirito's feminine looking GGO avatar or "Kiriko", is extremely popular in the fanbase. Expect any AU with a genderflipped Kirito to use said appearance and even name. The amount of fanart for him/her rivals that of the standard Kirito, with some fans even jokingly claim Kiriko to be their favorite SAO girl. Kiriko ended up becoming Ascended Fanon of sorts with him/her being a separate avatar for Kirito in Fatal Bullet and a playable character in the PvP mode.
• Natsuki Aki, Kirito's personal nurse in charge of his rehabilitation. She was first seen in the Phantom Bullet arc where she already garnered a lot of fans. Her return in Alicization was met with joy.
• The one particularly psychotic Laughing Coffin member, seen during the Knights of the Blood Oath's raid on Laughing Coffin HQ. He manages to kill three members despite being surrounded and his HP already reduced to red. He was an especially memorable character for how brutal and batshit crazy he was, giggling madly as he stabbed others to death, with lots of viewers commenting on him and singling him out.

Debuting in Ordinal Scale

• Yuna only appears for this film, but is insanely popular among fans. In fact, it's hard to find someone who doesn't like her, due to the fact she's an AI idol who has given some of the most beautiful songs seen in the series. It was saddening for many fans to see her go. However Popularity Power and her unique status as a Canon Immigrant has effectively allowed her to return as of the Alicization anime.
• Eiji has quite a large fanbase. He's easily one of the most sympathetic villains in the entire franchise, with him being Driven to Villainy out of grief. He's also depicted as a dark mirror to Kirito, with the two boys having similar histories in SAO, only with Eiji having a less happier outcome. There's also those who like him just for being a pretty boy. It seems his popularity has been acknowledged and he's made a full Heel–Face Turn as of the Alicization anime and joined the good guys.
• Ain-chan. The drone that Yuna rides on while singing and serves as her mascot. It remained unnamed for an extensive period of time, where fans constantly requested for its name, before finally Miki, Itou, and Kawahara decided to name it in a Behind the Scenes interview for Ordinal Scale.
• The unnamed Ordinal Scale player (who's apparently called "Tosaneko" according to this) with a humanoid tiger avatar has a lot of attention due to how much he stands out, curiosity over how he acquired said avatar, and his fighting style of using a rocket launcher.

Debuting in Alicization

• Young Alice Zuberg. While Alice Synthesis Thirty doesn't really count given she's permanently joined the main cast, the less featured Alice Zuberg, the Childhood Friend of Kirito and Eugeo, is still insanely popular. Taken away from her home at the age of 11, she was kind hearted and bubbly, with Eugeo viewing her abduction as My Greatest Failure. She was never allowed to grow up given she was forcefully extracted by the Synthesis process and replaced with Alice Synthesis Thirty.
• Selka Zuberg who is Alice's little sister gained a lot of popularity for her cute design and a Nice Girl who looks after Kirito. Her return after a Time Skip only sought to make her even more popular due to her Undying Loyalty to Alice and deeming her one of the best little sisters in anime. Her popularity within the fandom is strong enough, that she was announced to have her own arc in the game with her as The Hero, while Alice takes the back seat for her instead.
• Johnny Black. Despite being a member of the Big Bad Ensemble in both Aincrad and Phantom Bullet, is ultimately a very minor character who is barely seen. He returns as a One-Scene Wonder at the start of Alicization, but he sells his one appearance for every cent it's worth, hamming it up big time and channeling his inner psycho, as well as making the entire arc possible to begin with by poisoning and paralyzing Kirito.
• Sortiliena "Liena" Serlut, Kirito's senpai in Swordcraft Academy during Alicization, beloved for her pretty design, being a badass Action Girl, and how kind of a mentor she is to Kirito.
• Ronye and Tiese, the valets of Eugeo and Kirito respectively. These two girls garnered a tremendous following the instant they were introduced, with fans falling in love with how moe and cute they were. The amount of fanart existing of these two is staggering, to the point where someone unfamiliar with the series would mistake them for Main Characters. SAO merchandise is all too aware of this and has been marketing these two just as heavily as any of the other main SAO girls. Moon Cradle would turn them into Breakout Characters, with Ronye being the overall protagonist of the interquel.
• Cardinal. A Little Miss Badass who is Wise Beyond Her Years and the Big Good of Alicization, who is overall deeply loved by the fandom. Her appearances are limited, but she has huge bearings on the story.
• Fizel and Linel. The two mischievous Token Mini-Moe Integrity Knights who are the embodiment of Cute and Psycho.
• The Central Cathedral elevator operator (later named Airy) who is briefly seen in Episode 16. Fans' hearts went out to her given she has one of the saddest and most pitiful existences in the Underworld. For 107 years, she's done nothing besides operate the elevator, eat and sleep. She's even forgotten her name and the only thing she wants to do if she's released from her Calling is fly the elevator through the sky. She later gets an expanded role in Moon Cradle where she adapts to a life outside operating the elevator.
• Bercouli Synthesis One is very fondly remembered for being a very kind and honorable Integrity Knight, even to his foes and acts as A Father to His Men. He's also popular to meme due to the whole bathtub scene and wearing a bathrobe all the time, making him a "man of culture". War of the Underworld only sought to make him even more popular with how much of an absolute badass he is, showing why he is Commander of the Integrity Knights.
• Renly Synthesis Twenty-Seven the cowardly Integrity Knight is very popular, especially among light novel readers. He almost has the reversed scenario of Eugeo, being the meek and gentle boy between the two best friends, but instead being the one who lived.
• Iskahn, leader of the Pugilist Guild from the Dark Territory. A Bare-Fisted Monk and Blood Knight who just wishes to fight stronger opponents while getting stronger himself. He also pulls a Heel–Face Turn and sides with the Human Realm, even falling in love and later marrying Sheyta.

Video Game Continuity

• Zero from Hollow Realization, who was derived from an AI set up on the internet who would interact with fans and learn from responses they taught her. She has a lot of amusing dialogue such as "Official waifu status will one day be mine..." or "The four emotions I can currently experience are happiness, anger, sadness, and curry."
• Lievere from the Fatal Bullet DLC story line. Most fans agreed she made for a far more interesting villain than Itsuki with a more compelling story line than the one offered by the main campaign.
• Eydis Synthesis Ten from the Rising Steel/Unleash Blading, has also garnered a big fanbase despite debuting in a free-to-play mobile game. Her cute and cheerful nature, her devotion to Alice, and her cool weapon, instantly made her a fan-favorite. Being voiced by Kana Hanazawa does not hurt in the slightest, either. She was popular enough that she even got ported to Memory Defrag another mobile game as a playable character. Many fans wish for Reki to make her a Canon Immigrant and be made into the canonical ninth Integrity Knight.