Woah!

Allaire said:
Woah!

My thoughts exactly.

And all this time I've been thinking that Mystia couldn't catch humans.

Probably a human from outside. One scenario is that he's still relatively fresh into Gensokyo and so still sticks to modern clothes, but made friends with Mystia and now developed mutual attraction.

...Another possible scenario is that Mystia allured an outside human to eat him.

Ahem. "Eat".

Hmm. Not revealed in Pixiv description.

Adding to Speculation Fuel pool then.

Look over there! A bird and a human!

Mystia: First one of her class to graduate into boys.