@S1eth Do you have any sources for your recent edit? Like an artist's commentary or something.
This post is no longer available at pixiv so we can't check the tags there. And I'm not sure that we can make a clear decision based on a somewhat ambiguous skirt shape alone. Yes, it seems to be a bulge, but it is not necessarily a bulge. I've seen enough misleading art to be wary and not believe things so easily.
Text below the image: これは表紙って言うかそのイラストですが、妖夢な本をだします。 半霊がなぜか男の娘です。あとセーラーです。 Translation: Here's the illustration that I put on the cover of Youmu's book. For some reason her ghost half is a boy-girl. She's also wearing a sailor uniform.