I'm so passionate about otome MCs (my beloveds) so I have many thoughts. Apologies in advance.
I have to (and usually do) love MCs so I want her to have a set, defined character. She is the protagonist and POV character we're always with, so I think it's the most important that she, out of all the characters, is the most compelling, interesting and human (with vulnerability and complexity and acknowledged flaws etc). I like when the writer specifically crafts all parts of her character and all her actions, thoughts and dialogue to fit and enhance the story and themes.
I don't mind what personality or character she has, as long as it exists and it fits this particular story, because of course different MCs would fit different stories depending on what the writer wants to convey. This means that I am fine with (and usually still really like) even what is viewed as the "typical" Japanese otome MC that's demure, passive and kind as long as it's well-written (not written that way for the purposes of making the character less prominent, but instead as a specific meaningful choice), and if it makes sense and fits this specific story. Like in terms of kindness, although I do like morally grey and selfish characters (and this isn't incompatible with kindness anyway), I feel like it makes sense for the MC to also at least be kind to the love interests at some point down the line for me to believe they like each other and are close (because sometimes if the reverse is true and she's portrayed as too rude/mean, I can find it difficult believing her and the love interest even like each other, but again, everything depends on how it's done).
Basically, I'm fine with anything as long as it's done well and respect is given to the character as a representation of a real person - if the traits (no matter how "generic" they may seem) are not just there to be a placeholder, or because the writer thinks that makes the MC more generic for self-insert purposes. But are instead actually given attention as real traits a real person could have and are important to the development of the story, relationships or whatever theme is being conveyed.
Of course, just an MC having a character doesn't mean that I will necessarily like them, but the bare minimum (for me to even potentially like them and the overall story/relationships) is for the character to exist (as a realistic depiction of an individual person) and to make sense within the story (logically make sense with the setting and events of the story in universe, and also in terms of what type of story the writer wants to tell out of universe), rather than only being written as a vessel for wish fulfilment.
For me, immersion isn't when I can self-insert (because I never self-insert - but even if I did, the following would still probably make me feel more immersed and instead help with self inserting) but when the MC fits and enhances the story and setting, which requires for her to be defined enough in a certain way. Being a blank slate (which makes her not feel like a person) or acting in ways that are psychologically illogical (which can happen if the writer tries to reduce her personhood or doesn't explain her feelings or actions/doesn't let her react to make her more blank) or don't make sense for the story are the things that instead break my immersion. I need her to feel real, so I like when the writer actually respects her personhood and clearly shows her feelings and thoughts and how they are affected and change throughout the story. I can't be invested if she's just portrayed as a brick wall that exists only for fantasies and to allow for player interaction with the love interests, because then I feel like there isn't a proper dynamic for a believable and emotionally impactful relationship to develop (where both parties exist as people and thus can affect and play off of each other).
I like when her feelings and thoughts are strongly portrayed in detail and explained well, and when her voice permeates the entire story through how she describes things, tells the story and sees the world. I like monologues and when her feelings come through in every part of the story. I think this is also why I do end up having a preference for MCs that are shown to have vulnerability because it feels more human (and usually results in more introspection and insight into her feelings in the writing), rather than MCs that are only portrayed as super competent and confident without ever really losing or stumbling or having doubts, because it feels too idealised - like there isn't anyone who is perfect or that can perfectly handle everything, so it feels less interesting and realistic (like where are the stakes and what can be developed further if things are already perfect). In fact, I find it more interesting when the point of view character is put in dire situations and struggles that they can't just easily overcome, because it's usually more compelling to see how they react and cope (and potentially snap) under this pressure, and it feels more rewarding when (if) they actually do overcome it, or at least interesting to see how they are changed by the experience.
I also prefer defined MCs that are less customisable because then the writer can fit her character to the specific story they want to tell, resulting in the overall story being stronger in the message and content they want to convey. I feel like too much customisation can make the story more generic and have less depth in my opinion because the writer isn't allowed to commit to anything or explore any particular thing in depth (which I think is the main point of a story to me - to explore a particular theme and perspective in as much depth as possible) which means they can't be as specific with the themes they want to express.
I just think the best stories for me are the ones where all elements are meticulously crafted with love and precise intent by the writer (to contribute meaning to the story), and this would also (especially) include the protagonist (MC), which is why an MC that is meticulously crafted and respected as a defined character is my preference. This is also why I love when she has a set design and portrait, a default (preferably voiced) name and being voiced herself would be nice too (of course these should all be meaningfully decided so that they fit the story), since I like when they feel like a real character just like the others. This also allows the creators to craft these things to contribute meaning to the story - not just making them fit on a basic level, but also adding things like symbolism through the design or name. And like, if everyone else has a proper design but she has no eyes or something, or otherwise has way less character than everyone else and is basically just treated like a non-entity rather than a real (fictional) person like the other characters, then that just ruins my immersion because stories are meant to be at least passably realistic for the emotions to resonate.
Basically, she's the main character, so I'd like it to be her story more than anything else. I always hope to love her the most.
