Fans generally agree that Hunk gets the least focus and plot relevance of the paladins. It reached its peak in Season 2, where what little screentime he had resembled Flanderization. Many of his scenes revolved around food, his mechanical engineering skills were often forgotten unless necessary to the plot, he freaked out visibly more than any other paladin, and he was relegated mostly to comic relief, in stark contrast with the Hunk that learned to temper his caution with bravery during the Balmera arc. Later seasons brought emphasis back to his scientific knowledge and toned down the food and toilet humor, most notably getting Character Focus in "Omega Shield", but he continues to remain in the background most of the time. Fortunately, he did get more focus during Seasons 7 and 8.
King Alfor's hologram was introduced in the first episode as something of a Spirit Advisor via technology to Allura. Nothing is ever done with this except to bring him back to be killed off. Even for an intended Sacrificial Lion he was woefully underused.
Thace gets some criticism for not actually getting to do much when The Stinger in Season 1 and some of the Season 2 trailers suggested that his betrayal of Zarkon would be a major running subplot. As it turns out, he doesn't really get to do much with his status as The Mole, and the episode where he gets the most significant screentime ends with him making a Heroic Sacrifice before he has the chance to get any proper characterization.
All of Lotor's generals suffer from this to various degrees, as we get no insight into their backstories and their pasts with him, and their characterization is minimal for all of their screentime. Narti is hit the hardest with this trope, as the most mysterious of Lotor's generals, who has the fascinating power to possess people and somehow obtained Haggar's cat, Cova, which she uses as a familiar. She ends up killed by Lotor after he discovers that Haggar is using her as an unwitting spy. Acxa, the second most mysterious of his four generals, is mostly characterized by her loyalty to Lotor, but only appears in two episodes of Season 7 with the first establishing her sudden Heel–Face Turn and becoming an ally to Team Voltron, and doesn't appear again aside from a brief shot in the season finale as part of the Coalition. Ezor and Zethrid are reintroduced in Season 7 as a pair of rogue warlords, and seem to be on their way to becoming Rounded Characters as well as truly menacing villains, before getting apparently killed off in the episode where they return, though in the following season, they turned out to have survived the blast.
Narti is hit even harder by the fact that the other three abandon Lotor after he kills her, but they seem to forget about her not too long afterwards. She's never mentioned in the following seasons, and her teammates eventually go back to following Lotor despite their previous reactions to her death, though she does get mentioned in the final season.
One common criticism of Season 7 is lack of expansion on Adam's character. As Shiro's ex-boyfriend, a confirmed LGBT character, and someone who had been on Earth with the Garrison during the events of the show, some thought that he could have had fascinating character development and interactions with the other characters. Instead, he only appeared in two scenes and was quickly killed off with little fanfare, with Shiro barely grieving for him.
Romelle is brought back to help stop Lotor, has one adventure with the Paladins and then just kind of sits around in the background for much of Season 7, without even showing much reaction to the fact that everyone from her colony has disappeared as a result of Haggar's machinations. She has somewhat more to do in Season 8, although since it's the final season, it's still less than would be ideal.
Captain Olia has a cool, Sherlock Hound-inspired design, a group of family photos on her console hinting at her being a badass Action Mom, and is an Ace Pilot to boot. Unfortunately, her only speaking role is during the Season 4 finale, with her cameos in Seasons 7 and 8 being completely silent, serving as largely a static background character.
A lot of fans were left disappointed that Rolo's fate was never mentioned, since he's a recurring character throughout the middle seasons and his sidekicks Nyma and Beezer continue to make cameos into the end, but he himself is completely absent in Seasons 7 and 8. Some fanart by one of the show's lead animators implies he may have been Killed Offscreen but otherwise nothing is said about it one way or another.
Even Nyma isn't safe from this, as her role was relegated to cameos in the final two seasons.
Several fans felt Luca should've survived beyond the season 8 premiere and being a potential rival to Allura, Pidge and Romelle. Heck, there are fans who felt Merla could've been a potential rival to them and also feeling that she could've been connected to Lotor like her original series counterpart in addition to some feeling Ven'tar could've been a great ally to Team Voltron had she survived the destruction of her planet caused by Zarkon.
Even with the fact that his wedding to Shiro was a last-minute Author's Saving Throw some fans wish Curtis got more characterization, given that he is one of the more distinctive recurring background characters and has a name but it's never being mentioned onscreen. Given his numerous appearances they had many opportunities to expand on him that were never taken.
A lot of people were more than unsatisfied with how the writers treated Shiro's clone who stayed with the paladins froms season 3-6 and was established as a kind, good-hearted, if sometimes harsh leader, who had no idea he was being used to spy on the paladins by Haggar. Many fans hoped he would stick around, grow into his own person and/or become this reboot's version of Ryou Shirogane from the original show. Instead, the writers have Haggar forcibly turn him into her attack dog at the end of season 6, after which he gets his arm cut off by Keith, has his body used as a vessel for Shiro's soul, gets dismissed by Shiro as "that thing" and "evil" and is neither mentioned nor mourned by the paladins after the fact. What arguably makes this even worse is that it's heavily implied throughout season 5 and the first few episodes of season 6 that the clone was noticing there was something wrong with him and actively reached out to Lance for help in "White Lion".
Some fans felt that killing off Bandor ruined any potential of him interacting with Team Voltron like his original series counterpart did.
The Robeasts - one of the biggest draws from the original show - appear only sparsely in the first two seasons, and are then dropped altogether.
