The biggest reason for the bashing of Kana is basically that people are self-inserting themselves into Aqua’s shoes and focusing on what a perfect waifu would do instead of engaging with Akasaka’s work in the way he intended for it to be consumed as Oshi no Ko is big on confronting the realities of the misogyny idols and people who work in the Japanese entertainment industry face regarding waifus, which includes idols, being seen as existing solely for all the lonely men out there who feel unfulfilled in their lives, not having their own agency outside of being emotional support for these men and thoughts that are their own.
As for how this relates to Kana, many people who have trauma have a hyperfixation on their own thoughts, actions, and behaviours due to how they’re scared to relive their trauma again, and I know this ’cause I was traumatized myself, and this is referred to as hypervigilance. This can lead to them being very focused on themselves since they had no one else to look out for them when they were younger, not ’cause Kana is an abnormally self-centered person. In the case of Kana, her mother emotionally abused Kana and abandoned her after her popularity decreased as a child actress, which led to her having an inferiority complex and having a fear of abandonment, a reason for her intense desire for validation and feeling needed, which Aqua exploited by roping Kana into joining his sister’s idol group.
Aqua imprinted on Kana since they were kids ’cause he knocked her off her high horse when she had an overinflated ego, and that moment led to her on some level viewing Aqua as this amazing guy since it was a good learning experience for her that changed her life, and she rightfully realized she needed to improve given her self-esteem was tied to her acting ability. Then Aqua did all he did in the Sweet Today arc just to make her shine, and she became obsessed him thinking of him as her Prince Charming and made him the one she would work to have her feelings of validation and of being needed be realized, which is why she was so easily pressured into joining B-Komachi. Kana basically replaced her desire for her mother’s validation for her desire for Aqua’s validation.
And so Aqua avoiding Kana despite her doing what she’s doing for Ruby to seek validation from Aqua led to a serious decline in her mental state as she is someone who seriously needs therapy. In Episode 28, Aqua doesn’t respond verbally to Kana, who deep down wants his validation even if she realized at the moment that he was in distress, and he physically injures her too, knocking her to the ground.
Thus, It actually makes 100% complete sense why Kana acted the way she did since she made it her life to seek Aqua’s validation, and him telling her to shut up and strike her is basically seen by her as Aqua reproaching her, triggering her heavily in the process. As she was basically reliving her trauma all over again, you can’t expect her to be in-tune with Aqua’s distress and react accordingly. When people engage with animanga or any story in general, they should try seeing the behaviour of the characters from their own viewpoints and not that of just the protagonist that they’ve grown attached to. Kana did nothing wrong in the way she handled the situation, and it was the best she could given how serious her issues are. For Kana, Aqua telling her to shut up and strike her is basically denying her entire existence, so of course she’d shut down mentally and hyperfocus and bring up something that is congruent with her trauma in that she believes she is someone who doesn’t deserve the affection of others due to the emotional abuse she suffered at the hands of her mother. That she didn’t think of Aqua’s distress and focused on herself is ’cause instead of Aqua being her source of validation as she had intended for him to be, he’s now instead forcing her to relive her trauma, and her fight-or-flight response activated, a primal reaction to perceived danger. All of this shows that Akasaka understands trauma very well in how he has written his characters, and Kana should really not be getting any flack by people self-inserting themselves into Aqua’s shoes.
Kana acting in this way as an idol, prioritizing her own thoughts and well-being, led to people who see idols and anime waifus as having no value aside from being emotional support for themselves or the protagonists they self-insert themselves into being shocked as it’s a very un-idol or un-waifu thing to do, which is what led to the bashing even though it is not how Akasaka intended for people to view his story and Kana’s reaction.
As for why Kana doesn’t look for validation through friends, a lot of people with a bad childhood don’t have a secure attachment with at least one parent. To sum this up, attachment theory posits that humans are born with an innate need to form close emotional bonds with caregivers for survival and emotional security, and this influences childrens’ emotional regulation and relationship patterns, secure, anxious, or avoidment attachment styles even into adulthood. This is the reason it’s believed that many people with poor childhoods end up having problems with emotional regulation. People with anxious or avoidment attachment styles tend to look for partners who will trigger their trauma even if from the perspective of a normal person it seems insane since it’s an effort in part to gain mastery over their trauma by hoping they can change relationships with a similar pattern as that which initially hurt them.
Kana most likely got into acting in part due to her mother seeing her talent and wanting to validate her AND make money from her daughter’s talent, so Kana who ties her acting ability to her self-esteem, saw her mother as being the one who ensured that she got validation. But her mother emotionally abused her and abandoned her, resulting in Kana developing an anxious attachment in which she will look for a partner who will give her validation even if he isn’t attentive to her needs in general or will abandon her at some point just like her mother did.
Aqua giving Kana validation by putting a spotlight on her during the Sweet Today live action adaption made Aqua step into the role that her mother was in in that he was the one who recognized her talent and got her to shine. Even though she deep down knows that Aqua will not be good for her due to him being emotionally distant and not all that interested in acting, meaning there probably wouldn’t be too many opportunities for Aqua to put her in the spotlight given she deliberately avoids doing so after learning that people skills are more important to talent in the industry, showing that he’s the type who will trigger her trauma.
Many people with trauma such as this will focus on the person they want validation from, even if it goes against what they actually desire in their career. Kana’s primary focus may be her acting career, but she will be an idol for Aqua since it makes her feel needed by him since it’s for Aqua wanting his sister’s idol group to blossom. It is very common for people with an anxious attachment style to gravitate to a person with an avoidant attachment style like Aqua has even though the dynamic is toxic, but such is how things work.
Kana is incredibly well-written and is who I consider to be the best-written character in Oshi no Ko for being extremely consistent in the way she is written, and it should be the case that Akasaka either knows a person in real life much like Kana or he has read many stories from authors informed about trauma to depict her struggles so well in making her seem so much like a person from the real world. I applaud him for doing so well in making Oshi no Ko a compelling story with excellent characters such as Kana.