Due to its monthly release structure and strict narration format, Animorphs had its fair share of characters who feel underused.
- From #16 The Warning is Joe Bob Fenestre, who is best described as what would happen if Bill Gates and Hannibal Lecter had a son. He's a Self-Made Man who founded his world's equivalent of America Online (which due to being set in the 90s is much more relevant and powerful than AOL is today) and is notable for being one of the earliest antagonists to present the Animorphs with a case of the Grey-and-Gray Morality that would ultimately become a running theme in the series. Despite having a very ambiguous ending which fully allowed the possibility of a return, he never appeared again.
- From #18 The Decision is captain Samilin-Corrath-Gahar, a traitor, voluntarily working for the Yeerk Empire, even though he's not a controller. His actions are the only follow-up to Alloran's vague warning about the Yeerks having infiltrated the Andalite homeworld,, the only Andalite to have changed sides that the books introduce, but his motives are never explained, and he is killed early on in the very same book.
- From #38 The Arrival is Arbat-Elivat-Estoni, the Andalite brother of Alloran-Semitur-Corass (the host body for series Big Bad Visser Three). Much like his brother he's a Well-Intentioned Extremist, and his story presents some very interesting parallels with those of his brother. Much like Fenestre above, he gets an ambiguous ending which, while much more grim than the above (he's last seen alone, wounded and facing down a horde of Taxxons) technically made his return a possibility. Of course, he never did.
- David might count as well, given that while unlike the above characters he does make a return (in the appropriately-titled The Return no less) it's a much less climatic story than fans were hoping for, and is generally regarded as a missed opportunity despite being a very well-written Mind Screw book.
- Visser One is an interesting case in that while she has a very solidly written story, she is still regarded as this because of her very truncated fall and demise. VISSER ends with her being assigned to command a new invasion in the Anati star system, being promised a full pardon from her crimes if she succeeds and death if she fails. Ten books later we are told that she has returned to Earth, having failed and being sentenced to death. We are never told what happened in Anati or why she was brought all the way back to Earth just to be executed, which many fans consider a horribly missed opportunity. Several fanfics titled some variant of "The Anati Chronicles" have even been written just to explain this oversight.
- The Auxiliary Animorphs. They presented an opportunity to give disabled people a bigger presence in mainstream fiction and let them be the hero without needing to be cured (morphing can heal acquired disabilities but not lifelong ones). Instead, they ended up being a Redshirt Army who only lived for a few books, and even their brief tenure still focused on the six main characters. Maybe if they hadn't been introduced so close to the series' end.
- Multiple Right-Wing Militia Fanatic groups are mentioned in the epilogue... and that's it.
- Loren has an incredibly emotional reunion with her son Tobias and gets morphing powers herself during her rescue, and then mostly just sits around for the remaining few books.
- Toby Hamee is the Hork-Bajir seer. She's the great granddaughter of Dak and Aldrea, and personality-wise is far closer to Aldrea because she is much more ruthless than Dak ever became. She was an incredibly effective general, and had more long term plans than any other character. And she showed up about six times throughout the series.
- Rachel's little sister Jordan feels like she could have been better involved in the plot, especially given that she does seem to sense something's up at one point and asks Rachel to trust her.
- Chapman's daughter Melissa starts as Rachel's good friend who then becomes distant due to her confusion at why her father is suddenly so distant from her in turn, but her importance quickly fades, and she even suffers the indignity of her final appearance being in an alternate timeline that's wiped from existence at the start of Megamorphs #3.
- Aftran. Quite a few fans feel Aftran could have been a valuable ally if she’d been given another nothlit form, like a bird of prey, putting her in a Chee, or even if they’d found a way to swap her with Tom’s Yeerk. True, going to feed would’ve been dangerous but it’s not the easiest thing to tell them apart as Cassie showed, and the Chee are known to generate kandrona. Even just making her a bird nothlit would have given Tobias a helper for his aerial observations.
- Many fans were fascinated by the two animals who gain morphing powers in The Hidden without having the intelligence to do so properly. People would have liked to see one or both of those characters developed across the series, but they only appear in that one book and at least one of them dies in it.
