I haven't taken my meds for 2 days but I'm doing fine.
The reason why Suisei likes Miko so much is because she's into the type of person she can banter with, she likes teasing and being teased and Miko is the perfect holo member for that. Also someone who can make her laugh, like in this clip where she talks about her friendship with Toko there are many parallels with her relationship with Miko, including Toko making Suisei laugh so hard she can't breathe
https://youtu.be/yVTiEZRUYagPay special attention to the part where Suisei talks about "genuine teasing", something she can do with her closest friends because there's also affection involved that shows a sense of boundaries. Of course this is naturally the product of years of closeness, and while the real extent of Suisei's friendship with Miko can only be known by them since the viewers can only see the public stuff, it's nice that Miko seems to be around best friend tier with Suisei at this point despite having gotten close just some months ago, they way they interact on stream is a good indicator of that
but it could also mean that best friend is the most Miko will ever be to Suisei 