Because of Undertale's large cast, some characters were not given enough screen time, but it didn't stop people from appreciating them nonetheless.
- Muffet. Her adorable and well-done design, interesting personality, difficult yet fun and interesting boss fight, and great song for her battle make her a lot of people's favorite minor character. There's just as much fan art for her, if not more, than Toriel, to the degree that someone unfamiliar with the game could be forgiven for thinking she plays a much larger part than she actually does. Like Gaster, there's no shortage of Undertale fanart and fan fiction that upgrades her to main character status. It might help that she's designed by Michelle Czajkowski of Ava's Demon fame.
- Everyman, part of an attack done by one of the Amalgamations, is also becoming surprisingly popular, especially after being given an official name by Toby, despite being no more than a bullet in another enemy's bullet hell. It's even considered an icon of constant speculation source Dr. Gaster. In fact, it makes various return cameos in Deltarune.
- "Burgerpants", the cashier at MTT Burger, has gotten a fair amount of love for his desperate and miserable demeanor and goofy expressions, on top of being a character that those who work in the fast food industry can relate with. On top of this, he not only has the honor of being one of the very few characters who aren't scared of the Fallen in the No Mercy route, but he also goes the extra mile and actually puts them down in hilarious fashion if threatened.Burgerpants: Sorry, little weirdo. Threats won't work on me. I can't go to hell. I'm all out of vacation days.
- The Royal Guard Dogs in Snowdin, but particularly Doggo (who has been very popular with the Furry Fandom) and the Lesser Dog (for his neck-stretching antics). Even Toby seemed surprised
that Doggo became so popular. - Ice-E, the mascot character for Monster Kidz Word Search, has enormous popularity in comparison to his relevance (appearing in one scene as a picture). Like Everyman, he too would show up again in Deltarune, this time as a pizza mascot well-known among the Light World's cast.
- The Temmies appear in only one location, but are widely popular for their cute design and adorable quirks, often being called the Mr. Saturns of Undertale. The shopkeeper Temmie is especially popular for both providing the best armor in the game and their Surprisingly Creepy Moments.
- Woshua, who's just a random encounter, but is popular among the fandom due to his amusing speech patterns, character gimmick, and some hilariously dark dialogue, which have caused him to be regarded as a self-appointed memetic Moral Guardian.
- Snowdrake is another random encounter who's quite popular due to his adorable design and So Bad, It's Good puns, as well as his backstory making him a huge Woobie, as well as being one of the most tangible examples of Video Game Cruelty Potential if you choose to kill him.
- Rather coincidentally, all three of the "fire elemental" NPCs count. Grillby is loved for being one of the few named minor characters, having a unique design, and there being plenty of room for fans to develop him (particularly with his relationship with Sans). Heats Flamesman is liked for providing a random, hilarious moment, even getting a flair on the Undertale subreddit. And the fire girl in a Sailor Fuku in Hotland is liked just for being adorable.
- Perhaps the strangest example out of all of these, the Yellow Soul, due to the prevalent "yee haw" meme on Reddit. It's to the point that there's an entire fangame devoted to them. It even has a wiki.
- Of the side characters who don't have a bearing on the plot, Napstablook is adored for being huggably sad all the time, having a jazzy boss theme, saving the player from the Mad Dummy, and the weird yet awesome moment that happens if you choose to hang out with them.
- Madjick also has a bit of a following, seemingly for no other reason than it was a running joke that somebody started.
- The Annoying Dog, who most players will find more amusing than annoying due to the ways it trolls the cast, such as apparently being one of Papyrus's death traps and sneaking into the player's inventory and then absorbing a "legendary artifact" when deployed.
- The Tsunderplane, for being a parody of tsunderes. Not that it likes being popular or anything!
- If fan art is any indication, Endogeny is the most popular of the Amalgamates due to its unique design, along with the Creepy Cute charm of it being a Big Friendly Dog in the form of an amorphous Eldritch Abomination. The game even lets you play with it!
- Mad Mew Mew is an Optional Boss who's popular for many of the same reasons as Muffet, like her cute design, the battle mechanic unique to her fight, and her catchy battle music. It also helps that she's the new form of the Mad Dummy and the fight provides closure for their character arc.
- Dr. W.D. Gaster has quite literally no role within the game itself, has little personality to speak of, and is explicitly meant to be an easter egg for curious players, but his bizarre character concept and lore implications made him more popular than some of the actual main characters. It's to the point where he is often promoted to a major role in most Undertale fanfiction.
- Chara is a One-Scene Wonder who only appears in the game in person at the end of a hidden route with specific requirements, with little clear explanation of their personality or goals compared to other characters, and it's not explained how, why, or exactly at what point they appear to take over the flavor text and possibly the protagonist's body in a case of Demonic Possession. Yet, they've always made a big splash.
- Early in the fandom's life they were seen near-universally as a detestable and shockingly evil person in spite of their youth akin to The Omen's Damien Thorn, and perhaps the only irredeemable character among the cast, but still appeared in countless fan arts, comics, fanfiction, animations, and songs as the fandom's go-to villain in fan content and crossovers, and depicted as a tiny evil badass able to go toe to toe with Sans the Skeleton. Part of this can actually be chalked up to the influence of Sans's popularity, as the reasons why his fans liked him so much naturally demanded an equally powerful Arch-Enemy as the source of all his misery to battle, and, with Flowey closest to filling it in canon but too sympathetic for fans to use due to his identity as Asriel, and Frisk too widely seen as a gentle pacifist and Protectorate of Sans' to be pitted against him, Chara was the natural choice to take over the role.
- But, having sparked a great deal of discussion and analysis because of their mysterious nature and how their shadow loomed large over the entire game, the Narrator Chara theory that came months after the game's release hugely helped rehabilitate their image in fan circles by using all the flavor text in the game to, in one fell swoop: flesh out their character to have a cute, snarky personality with a likely sad backstory that caused their less-admirable qualities and development into a suicidal Well-Intentioned Extremist at such a young age; indicate an arc of quiet character development and an adorable Odd Friendship with Frisk in normal routes; and recontextualize the Genocide run into their tragic corruption into becoming their own antithesis. The combination of the "cute kid" fuzzies and heart-shattering angst naturally appealed to many, to the point that one 2021 poll of current Undertale fans on Tumblr had Chara come in at 3rd place as the most popular Undertale character, the highest-voted character behind the infamously popular skeleton brothers themselves.note
