I have quite a few, I'll add more later if I remember.
I like a lot of characters in general, so chances are 8 times out of 10 they're also a very hated character in the fandom too. Or at least disliked enough, even if it's not on the forums.
Sorry for any grammar mistakes.
Bramblestar:
The amount of visceral hatred he gets is kinda wild. It took one video and suddenly he's treated like the worst thing in existence when in reality he's between a rock and a hard place. He has the pressure of his father's terrible legacy always on his heels, just from even looking like him. I think people forget that Firestar himself once didn't trust Bramble, back when he was still a kit for StarClan's sake, and Goldenflower had to call him out on that. Later he had to kill his own brother and choose Firestar over him.
He wanted to have a close bond with a lost sibling, but things only went down hill, especially after Tigerstar sunk his claws into them again. Bramblestar was ambitious, but ambition isn't inherently evil, and he made the right choice in the end. But it was at the cost of his own brother's life, who he genuinely cared for. Then you have his mate, whom he genuinely loved, and had a healthy relationship with, but she lied to him. She used him. And people constantly say they feel bad for Squirrel, and that he should suck it up, which just goes to show their lack of sympathy for his side, let alone both.
He deserved better. Doesn't help that later he loses one of his daughters twice, and when he's trying to run his clan properly, his mate also deputy, constantly goes behind his back and breaks their trusts (AGAIN). Wanna make it worse? How about ex-friend that takes control of his corpse and terrorizes his clan, which eventually led to him getting early dementia and PTSD. And practially undoing all of his work and reputation he had to fight for in the past because other cats merely looking at him now only see the monster that was Ashfur's possession in his skin. Also his mate, again still undermines him and makes him look like a fool in front of other clans, which doesn't help either.
Give him a break.
Nightheart:
Just because characters are "ooc" by some people's standards doesn't make what they did right. And I'm kinda tired of seeing this excuse used for the mistreatment that Nightheart gets. Not to mention he's pretty much coded with the trans experience, and I guess I feel for him in that sense. I get wanting to be your own character, and not having all of it's worth being boiled down to your bloodline, or also the status of your direct family lineage. His previous name also, to this date, grated my nerves. And it's also annoying to see how characters reacted to him wanted to change it. The way the clan treated him like a failure, then to waiting for him to fail, and both times he's put on a pedestal he doesn't want and is forced to prove himself. I would want to run away too and start over. Even if another clan isn't all flowers and rainbows, at least they'd treat him as "Nightheart".
Bumblestripe:
Awkward guy likes a cat, and said cat constantly compares him to another cat he didn't even know she liked. She then becomes mates with him because he's convenient before dumping him like a bag of bricks. Turns out she was in love with the other guy the whole time and ran off to be with him, only to return with his kids later, but people expect him to be sweet to her after she betrayed their clan. It annoys me het gets slapped with the Ashfur 2.0 label just because of one dumb scene in a bonus chapter as if that suddenly excuses everything else that happened to him.
Nightcloud:
Husband approaches her and wants to be mates, she's down for that and wants to have a loving relationship with him, raise kits, and be happy. Like any cat in her position would want. Turns out he was just using her, and their child to seem loyal, and he had the audacity to still flirt with the other woman in front of her. While also emotionally cheating on her, and neglecting their son, doing the bare minimum isn't parenting. And then only after they break up, and another cat shows interest, suddenly he's mooning over her and jealous, and realizes how badly he fumbled, like seriously?
Scorchfur:
Rightfully angry about the state of his clan, is just more vocal about it compared to most cats. Yet he's the only one that gets most of the licks for it, and it annoys me to this day that people try to justify what Tawnypelt did to him. He voices his frustrations, and her response is to attack him, but when he attacks back in self-defense, suddenly he's the bad guy? Hunh? And even then he voiced regret and was willing to leave. There's also the moment he stood up to Darktail, or when he was still weary of Tigertwo being leader. He's shown to be loyal, a good father, and protective of his clanmates (like Dovewing of all people), yet people blame him, and even other cats, for the sole fall of ShadowClan, what?? He's sometimes pathetic, yeah, but I like that about him too.
Mapleshade:
The guy she was with later turns out to be public enemy number one. She lied by omission, yes, but did it justify the lack of leniency and terrible treatment she received afterward? No. There were SO many opportunities where Mapleshade's situation could have been handled better. During her "trial", when she was in the river, after she arrived in RiverClan, etc. And it didn't matter if she lied or not, because it's the mere fact the kits belonged to Appledusk that it set off that easily avoidable series of events. I have no sympathy for any of her victims because they showed her none before.
Breezepelt:
Father is a neglectful piece of fox-dung, openly also shows he's still in love with another woman in front of him that's not his own mother. Also berates and undermines him, while also refusing to acknowledge his accomplishments after he bends over backward trying to make him proud of him. Ironically his father praises his rival (half-brother from ANOTHER CLAN) more than he ever did him.
Father also encouraged him to hate on mixed-blooded clan cats at a gathering of all places. He ends up being manipulated (there's another term that fits better but I won't say it) by DF cats, turns on his siblings, and in the end, his father STILL doesn't take responsibility until an SE. An SE where he even thinks Breeze is better off dead, his own kid, which is just....ugh.
Berrynose:
I feel like this guy only gets hate just because he's got more of a personality than most cats. He's an over-confident, arrogant even, warrior who likes to throw around his weight, but he's also charismatic. However, no one ever talks about the other bits of his personality. Like his admiration for Lionblaze, or his absolute cries for help over Honeyfern. He's a nice balance, with a unique design, and I would have liked to see him become deputy at one point, but it does feel like he was wasted in the end by the writers, which is a shame.
Leopardstar:
Oh hey, a cat with an incredibly terrible up bringing, and has both internal and external struggles, is actually fleshed out well, and shown to be morally gray. However she shows ACTUAL development, and reasoning for why she did was she did in the past. Regret apologizes, and continued to change for the better while making her clan one of the best powerhouses of relevancy during her reign. What's crazy is that you can switch this whole thing with Blackstar, but only one of them gets so much crap for this in the fandom.
Jagged Peak:
I never understood the hate that Jagged got in the fandom? Was it because he didn't end up with Rainswept Flower? I didn't mind that pairing either, from what I remembered, but I didn't mind him being with Holly. At first I thought Jagged was annoying, but I liked that he became basically became a prominent background character, and him being brothers to Sky and Gray doesn't get talked about enough. Also Holly was an improvement for him, considering she gave him confidence.
You can call it arrogance if you want as well, which is fine, but considering how Sky threw him out and left him to die, and how Gray would always talk over him (I love you Gray but seriously?) Holly was exactly what he needed. He also was so right for calling out Sky when they spoke to their mother. And had the idea of the 5 petal flower sign down from the start but no one listened, hmm.
Needletail:
Rebellious radical teen runs off because her clan didn't give to scraps if she was dead or not in the past, LOL. And when she wanted the cat that helped her to join as well, they turned him away??? The irony of that. Suddenly they only care about her because she has importance to a possibly prophesied cat. Rowan's treatment of her, and the other apprentices never sat well with me, he really didn't know what to do with all of them. And the fact that older cats were also not doing their jobs either, the whole thing was a mess, so I'm not surprised it ended up the way it did.
However, Needle bit off more than she can chew, and I don't see anything wrong with her have a reality hit over Rain being killed. Especially when she wasn't aware of other things like some of Darktail's methods. People seem to forget that Needletail was fairly young herself and that she was more or less a wayward cat with a lack of actual guidance in her life. In the end, she died for Violetshine because she did love her like a sister, how you interpret their relationship doesn't change that either.
Half Moon:
I never understood why she got so much hate to be honest. I actually didn't mind her, nor her relationship with Jayfeather. And to be honest, out of the THREE and their romance arcs, I think Half and Jay had the best-written one, which is kinda shocking. I liked the idea Half Moon had an important role in the clan's history and what she had to give up for the future of the stars. Essentially without her and (ironically) Jay there would be no Clan. And I also liked that she didn't push her feelings onto Jayfeather's reincarnation either. She let him figure out how he wanted to feel about all of it in the end, and the idea of them finally being together again after he finally passes is sweet. It's really just a star-crossed lover x reincarnation lost love trope, it's very common in media, so I don't see the big deal. And Half Moon herself is nice.
Moth Wing:
I like her, I don't understand the dislike for her as well. I was actually able to get through her books, which is honestly a feat nowadays when you see other recent SE's. I think the reason she does get a lot of hate is that she made the no mates for medicine cats law, but honestly, I was never against that rule either, and I have no problem with it staying. Outside of that, I found her character to be interesting, the way she was written, especially in how she thought, was nice. And seeing her have realistic struggles made her character feel more down to earth, rather than on a spiritual level because of her position. I wasn't fond of her relationship with her mother, but I'm happy it worked out in the end, somewhat.
Daisy:
I believe most of the hate she gets isn't really on the forums but on other sites in general nowadays? She's a very sweet cat, well-rounded, and one of the best background characters who honestly deserves more credit. It annoys me that people think that her being a perma-queen = her being lazy. You can tell the only ones that make that argument are people not aware of what midwives do. Also, Daisy not knowing how to fight and defend herself early on, made sense, she's a barn cat, and she didn't need to know how to do that. I also disliked how the clan pretty much pressured her to join because they needed more kits, but that's a talk for another day. And even though she had feelings for Cloudtail, she quite literally never acted on those feelings, never was annoying toward Bright, never did anything?? Like she literally was even willing to leave simply because of that because she didn't want to get between them. Give her a break.
And yes she can fight now, did no one remember her bodying DF cats like no tomorrow to protect the nursery, cause I sure do?? She practically helped raise the majority of modern ThunderClan at this point. Her knowledge about raising and treating kits should be priority at this point. The other thing that I see sometimes is how she was acting in her novella toward Smokey, oh yes the Tom that continuously gaslit her, blamed her for things out of her control and was being unreasonable over his kits that could die? Right. Anyways. Daisy deserves better.
Flametail:
For the most part I liked Flametail, but I do acknowledge the inconsistencies with his character because of how he was written when he first joined StarClan, vs later when he was lost. I do think that based on the later interactions, him getting an omen, no one listening to him, and then later dying is a pretty good reason to be angry. Also, no one guided him in StarClan at one point (or he got lost??), and he almost got iced a second time, like, hello?? But he's not supposed to be bitter or upset about that. What's crazy to me is that a lot of problems that happened in the next arc after this could have been avoided if the Erins hadn't killed him off lol. Also to this day, it annoys me that he wasn't the 4th cat in the prophecy (it should have been him or Breezepelt tbh, but then again the prophecy didn't need a 4th) cause it felt like his meaningful moments with Jayfeather meant nothing after.