Of the four main boys, Lucas is the most divisive. Many fans love his Brutal Honesty and being the most logical member of the group. In Season 1 however, others found him to be the most whiny of the group as his initial distrust of Eleven was a constant source of friction among the kids (though that was probably the point). Even after he gives a heartfelt apology to Eleven, some fans still weren't entirely convinced. Season 2 makes great strides in showing that after Season 1, Lucas's Character Development has changed his views on letting newcomers into the group and he is notably the only one of the boys to be a true friend to Max. We also get to see more of his Adorkable side and home life. The clincher is when it's briefly shown that he has become much more chummy towards Eleven in "The Gate".
Mike became one starting in Season 2. While he remained a Nice Guy on many levels and was still a beloved character among fans, some didn't like his Jerkass Ball behavior, particularly his attitude to Max, Will, and Hopper. His treatment of Eleven in the third season also didn't help, even if some empathise with his stumbling given his youth, inexperience, and taking dating advice from Lucas. Not helping matters is Mike has consistently been considered one of the show's most underutilized characters after Season 1, with many claiming the writers don't know what to do with him now that the main kids frequently split up, leaving his original role as The Leader irrelevant. Season 4 deepened the divide, with some fans feeling that his role was reduced to being Eleven's cheerleader and that he was an uncaring friend to Will, while others felt that his communication problems and obliviousness to Will and El's struggles were realistic for his character and provided interesting tension.
Jonathan has fans who love him for his brooding, lone wolf persona, the development of his relationship with Nancy, and dedication to his family and especially his unconditional love for his little brother Will. Others find his character dull and stagnant, and dislike how Out of Focus he is past Season 2, arguing that he's mostly restricted to being Nancy's sidekick in Season 3 and palling around with Argyle in Season 4. There's also the scene in Season 1 where he takes pictures of Nancy in a state of undress without her consent, which some find unforgivably creepy.
Max is an odd character for a lot of fans, who felt she didn't contribute much other than snarking at Lucas and Dustin, as it wasn't even decided if she was going to believe them or not. She was redeemed a bit when she began to grow closer to Lucas and also finally stood up to her step-brother, who had been an abusive and controlling force of her life in Season 2. In Season 3, she develops a close friendship with Eleven which was highly praised. However some viewers still felt the character was underutilized, surviving little purpose except to provide a mouthpiece for the relationship drama between Eleven and Mike. It wouldn't be until Season 4 where she gains an important arc in dealing with Billy's death and narrowly escaping Vecna, which was seen by the fanbase and critics as one of the best parts of that season.
Billy Hargrove, Max's older brother and an antagonist introduced in the second season:
A big debate sprung forth over how sympathetic the character actually is due to being a victim of Parental Abuse. There are those who feel that this doesn't justify his own abuse towards Max and his misogynistic attitude, while others were hoping that he'd undergo a Heel–Face Turn arc like Steve did the season prior. The Duffer Brothers themselves stated, "It doesn't excuse his actions at all, but it at least helps explain them."
After his arc in Season 3, there was a lot of debate as to whether he truly redeemed himself in the end. Some fans who disliked or hated him became a lot more sympathetic towards him after Season 3, while others still felt he deserved no sympathy. There were also some fans unhappy with how much Max seemed to care for him in Season 3, after his abuse of her in Season 2.
Erica in Season 3. She was an Ensemble Dark Horse in 2 thanks to her bratty personality towards Lucas, and some fans enjoyed her joining the Scoops Troop. Other fans were less forgiving, as every line out of her mouth seems to be bratty and snarky, making her into a flat-out Jerkass rather than just an Annoying Younger Sibling, and they feel she doesn't add anything to the Steve/Dustin/Robin dynamic. While she was Demoted to Extra in Season 4, the redirection of her behaviour towards the Jerkass Woobies Eddie and Jason largely pushed her back towards her Season 2 status.
Jim Hopper in Season 3. He spends almost the entire season constantly agitated and pissed-off. Some viewers liked his attitude as someone who spends the entire season getting shit thrown at him and never getting any time to cool down, while to others he appeared as dangerously unstable, considering his willingness to threaten Mike with violence if he didn't break off his relationship with Eleven and the way he went full-blown Jack Bauer on the corrupt Mayor.
Robin in Season 4. Some feel she's been flanderized into an obnoxious Plucky Comic Relief who's done a complete 180 from her more snarky and level-headed characterization in the previous season. Others find her to be as relatable and endearing as ever and welcome her more dorky Motor Mouth personality as a progression of her learning to open up to the other members of the group besides Steve and grappling with trying to express her romantic feelings for her crush, Vickie.
Side Characters
Suzie due to her making Dustin sing "Never Ending Story" with her in the middle of Season 3's climax in return for her essential help. Detractors say she unnecessarily endangered everyone's lives in doing so and found the attempt at a cute moment inappropriate given the deadly circumstances our heroes were currently in. Others are more forgiving since she didn’t know anyone was actually in danger when she did this, and found it to be an adorable scene made hilarious by the concurrent danger.
Ted Wheeler. Some fans dislike him for being a collection of some of the worst tropes related to fathers, having No Sympathy for the kids, very few hints of depth, and being an all around tasteless, lazy jerk who complains all the time. Others like him due to finding his intentionally cliche and idiotically oblivious portrayal to be funny and note that for all of his clear faults, Ted is far from the worst father seen on the show, still providing for his family and caring in his own reserved way.
Argyle. Some fans dislike him for not really contributing anything new to the series and just existing to serve as a Plucky Comic Relief, a role that plenty of other characters in the series would be able to fill. The fact that his character is never really expanded upon besides being The Stoner friend of Jonathan doesn't help. However, others like him just fine and appreciate his friendship with Jonathan, while finding his stoner antics to be a welcome source of humor in an especially dark season.
Jason. Some found him sympathetic and believe his campaign to eradicate D&D was a misguided but genuinely altruistic effort, while others found him to be an unlikeable and deranged lunatic who became the very thing he swore to destroy.
Vickie. Some find her to be a cute love interest for Robin and enjoy their chemistry in the Season 4 finale. Others dislike how similar they are personality-wise and find their romantic subplot to be too minimal and a disservice to the series' first openly gay character. There's also a third party who argue that it's too early to judge her character since she's only in three scenes and are willing to give her a chance to develop more in Season 5.
