I'm always willing to help others, anyway. I love helping peeps with Touhou.
I'll gladly ask for your advices, then if you don't mind.^^ Maybe I should create a topic about it, but you'll see where I'm coming at when you'll read me.
Well, that said, I want to talk about Koishi's popularity. I'll just express my opinion, because in touhou, giving a general opinion about a character is somewhat hard, since touhou is a little based on interpretation and that's part of why you get all the fanworks. I think that people like Koishi because she got a sad background while being carefree.(well, technically, she's not even carefree, she's kind of "nothing" since she closed herself from the world and erased her ego) People can relate a little to this kind of character, and when you can relate to a character, it oftenly leads you to appreciate him/her. As for me, I like her because her story is sad, and her attitude in hopeless masquerade totally made me feel more sad for her, even though she was influenced by the mask of hope, from what she said at the end of the extra of SA to Reimu/Marisa, and from the behavior she had in HM, she really felt like she wanted to exist, to open herself, that she was interested in other people. But reading the mind of people made her suffer so much that it's really hard. And even though I haven't gone that far in life, I REALLY can relate to this character, most of any character in touhou. That said, my favorite one is still Parsee since 6 years, even though Koishi is in my top 10.
For a more objective side, I find it hard to understand why she's very appreciated. But Koishi Komeiji's Heart Throbbing adventure is almost finished, the last episode should be coming this year. It probably impacted many people, and while it was kind of messy at the beginning, it developped itself into something very deep and emotional about Koishi's background and story. I will not hide you that KKHTA is something that made me like Koishi's character even more, even though I already liked that girl. What is true for Rumia with Osana Reimu/Reireimu can also be true for Koishi with KKHTA.
That said...being able to stay that high, I also can't explain the reason. Of course being at the top 10 is explainable, but I agree that it's still surprising to see her that high. It's like she's appreciated like a third temporary protagonist in a shooting game.