this isn't gonna be super detailed since i'm in the middle of helping my partner move so let me know if there's anything else you want thoughts on specifically, but i wanted to help out a little anyway because i really like this character and i think she has a lot of potential!
-so you're saying she's your antagonist: is she like, the primary antagonist of your story? it might just be because i'm reading her entire arc condensed into one google doc, but the moment i started reading about her, i was waiting with baited breath for a redemption arc, and i was rooting for her and kacey to come to an understanding. beyond them being tennis rivals, and yannika coming across as sort of condescending and bossy, i'm not seeing what exactly she does to kacey to warrant her being considered the antagonist as opposed to just, someone kacey doesn't like and eventually grows to like (though knowing what i know about kacey, i can understand how their personalities would clash). i'm sure you've decided what direction you want to go with yannika, whether you want her to be a major or minor antagonist, but if she's a major antagonist, i think what you need to do is put a major emphasis on how condescending, passive-aggressive, bossy, and unkind she can be, especially to kacey, and obscure her more sympathetic qualities until she cries to kacey in the bathroom (which you might be planning to do already and i'm just not getting that because this is a character sheet and not the story itself lol)
-obviously this is her character sheet, not her moms', but i need to understand what kind of people they are. i'm getting that the one mom tries to buy her love, and the other mom is cruel and abusive, and that their main redeeming quality is that they aren't homophobic or transphobic. why did they adopt her in the first place: to seem like better people? because they felt like having kids is what they were supposed to do? how did they qualify to adopt her when the one mom is a drug addict, and how does she get the money for drugs? what kind of careers do these people hold? also, careful not to demonize addicts as you write them; "mom 1 is a shitty person who happens to be a drug addict" as opposed to "mom 1 is a shitty drug addict." why's she an addict, also? is she also bearing the brunt of her wife's abusive tendencies? i actually think yannika herself is really well-developed, but i need to know more about why she developed the way she did, which is to say, i need to know more about who her moms are, because as it is, they're reading more like convenient plot-device amalgamations than characters (which is fine as a jumping-off point, because that's how a lot of my antagonists start off whether i mean for them to be that way or not, but they have to develop into something more).
-this isn't really a hole in her description, just something i'm wondering about. does she have a lot of close friends, or just followers? does kacey hate how people flock to her? i know you mentioned she's popular, school is her happy place, she likes to be in charge, she dresses like a prep/jock/egirl (so pretty trendy). but is she both popular and lonely? is the reason kacey is the first person she opens up to because she's scared to open up in general, or also because she's scared to her friends specifically? i'm sure you have an answer to this already, i'm just curious.
anyway, i like her a lot! i've always had a soft spot for girls who act bossy/mean/condescending/cold girls to sort of cover up a deep-seated vulnerability, my character darcy falls into that category (and she also ends up with page, my plucky extraverted protagnist. i'm assuming yannika and kacey end up together? it's a dynamic that's very close to my heart). again, let me know if there's anything else specifically you want my thoughts on and i can try to get back to you about it later!