crease kazatabi_lee

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Is it really a crease? The original artist is not going to scan his own work and post it on pixiv
It is in the right place for a crease, but it's a funny-looking crease. It super-thin, incredibly uniform, blue (like the image), and cuts off right before it actually hits the top. But if it's not a crease, why on earth is it there? There's nothing in the image which looks like it would cause a line like that. So, it's just all around weird.
i think is just an effect D :
separating the two ;p