As season 4 and 5 premiered and more about the lore and main plot was revealed, plenty of fans started to feel this way about Adrien/Cat Noir. Despite ostensibly being one of the main characters and the fact that much of the series' backstory is centered on his family (with his father being the show's Big Bad), he gets far less focus than Marinette/Ladybug, who's usually the one to learn most of the lore while Adrien gets left in the dark or written into situations that keep him from learning these things.
While she's still a major Base-Breaking Character, a number of fans agree that Lila has been extremely underutilized, especially in season 3 and 4. In her first few episodes, it looked like she would take the Alpha Bitch role from Chloé after the latter went through a Heel–Face Turn, with her alliance with Hawk Moth providing a number of story opportunities. Instead, Lila never really had much involvement in season 3's ongoing plot, and Chloé ended up undergoing a Face–Heel Turn and being Hawk Moth's ally in the season finale instead. She gets even less screentime in season 4 where she plays a minor role in only three episodes. Season 5 finally corrects this, setting her up as a more important and mysterious antagonist who is set to take over Hawk Moth's role of Big Bad going forward.
There are numerous minor characters that fans wish would get an A Day in the Limelight/akumatization episode, such as Mireille and Marlena.
Sabrina. It's been over four seasons and, even though there have been some hints of independence, she still has yet to see her character grow beyond "Chloé's pet Extreme Doormat/lackey", much to the consternation of her fans. She did get some progress away from that role after turning on Chloé during "Confrontation", but it's largely seen as too little too late.
Many fans of Zoé Lee, Chloé's half sister, are rather disappointed that there are only two episodes that focus on her and would have prefered if she had a bigger role in season 4. Intrigued several fans with her existence alone — however, the writers seem to have no plans for developing her character beyond "the anti-Chloé", contributing heavily to the contentious reception she's gotten.
Su-Han is a fully trained member of the Miraculous Order that arrived in Paris specifically to track down the Miraculous Box and solve any issue its Miraculous may have caused. He has just a major role in his debut episode, where he acts antagonistically due to Marinette not knowing the rules a Guardian is supposed to follow, a rather small role in "Ephemeral", a cameo in the finale and gets Put on the Bus during the start of season 5. People had hoped for more interactions between him and Marinette.
Duusu the peacock kwami doesn't really get all that much to do despite how important his miraculous was for seasons 3 and 4. "Truth" implies that he has mood swings because of his damaged miraculous, but nothing comes of it, and he only appears once or twice. During season 4, his miraculous is fixed, and he reunites with Nooroo, but after one or two interactions with Nooroo, he just sits in the back. This is disappointing since his miraculous basically started the entire show (“Feast” reveals his miraculous created the Dormant Sentimonster which destroyed the guardians' temple and separated his and the butterfly miraculous from Master Fu, his miraculous is responsible for Emilie Agreste's coma and Gabriel Agreste's descent into villainy, and his miraculous is responsible for Nathalie Sancoeur becoming more and more ill) yet when he's back to normal Duusu himself never really reacts to or likely even learns about all the damage his miraculous has caused. Since kwamis want their powers to be used for good, and Duusu himself in "Hack-San" wants his powers to stop being used for evil, it's wasted potential to ignore how Duusu would feel knowing that his miraculous has caused so much pain to many, and is the reason his fellow kwamis are sought after.
A number of fans have felt that Nino should have been the temporary Cat Miraculous holder in "The Kwami's Choice" instead of Zoé, since his girlfriend Alya got to be the temporary Ladybug holder (though this one is justified, as Plagg specifically rejected having another pair of lovebirds to avoid a repeat of what happened with Ladybug and Cat Noir).
"Penalteam" introduces no less than four new heroes, with the drawback that no individual screentime or development is given to any of them, making it unclear why they were chosen as heroes beyond "we had yet to use these Miraculouses and these are Marinette's only classmates that haven't received one yet". Especially exasperating is that this is immediately before the season four finale, in which Félix hands all of the Miraculouses (sans the Ladybug and Cat) over to Hawk Moth, preventing the new heroes from reappearing for a whole season.
Emilie Agreste is pretty much the catalyst for everything that happens on the show, since it was her magically-induced coma caused by using the broken Peacock Miraculous to create Adrien that led to Gabriel becoming Hawk Moth in the first place. However, what little we know about her as a character starts and ends with admittedly biased sources who paint her as an absolute saint, the few hints that she may have had a darker side are never fully followed up on, and even the attempt at expanding on her backstory in "Representation" is relayed by a potentially biased third party, leaving her with little characterization and no purpose outside of how her disappearance/coma affects her loved ones.
