Thank you!! I try not to throw around diagnoses honestly because I just think most people, even other mentally ill people, are extremely ableist and I just don't want my characters to be viewed as behaving in a bad way because of their disorder or as "evil" because of having a certain disorder.
With autism it's hard. I think so many characters are obviously autistic or other disorders get ready as autism (like stpd, aspd, etc), and it's why I talk about infantilism a lot. It's hard to avoid infantilization in your life when you are so visibly autistic (or something adjacent to it).
Sometimes I write a bad person who is also just.. bad because of their unmanaged and/or poorly managed mental illness LOL... And it's fair for people to dislike them but I don't want them to contribute to poor perceptions of that disorder. Speaking of that, I love Shiori Takatsuki.

Smiles.. I'm happy to bring joy.
I do wish I had a method, but really, mm.. My characters may start out empty but they develop. Writing and development is my favorite part of creation. Even the designs, while they are cute girls, all have like.. reasons / things I think about.
IE. Aiyana was originally going to be a lolita, but I just kept thinking her design would make her small.. Tear dropped shaped. I thought of her as modest girl... Instead of big curls I usually give characters, I gave her long braids to add to her.. smallness. Her hoops are there because I associate them with the South ahah, but I gave her layers because I think she's most comfortable in them. I wanted to figure out how she lived but didn't think she was on government assistance. Translation culture was interesting to me at the time and it allowed for her to work from home..
I cut out the fat... I really love writing families but there's no room, so I tend to imply it, but you know, it's in my mind, how their family resulted in them behaving as they aged.. The way people feel about affection often influenced by family dynamics. It's on my mind a lot..
Mm and I like to make inspo boards too.. Though with WIILTT I didn't make too many. I listened to a lot of maidcore haha... I just kept thinking of Aiyana's and Emma's life as endless droning.. So I'd make these big melancholic maidcore lists....
That is just one example though ^^