It's one of his forms he uses the most, as not evryone can see it, it's also just how his actual shadow look like, he can use it just like Alastor does, it can also reveal some of his intent, he mentions it as his, and I quote "hunting body" if you want the whole sentence here it is : "People often call it my shadow, but it's just my hunting body, as the third eye is here to focus on the prey and it's got a lot of advantage."also you still vote 1 ?

Oh well that explains some stuff about the mouth's design, but if you don't mind me asking, what's exactly Kiiro's shadow?

Yes but context is missing, Kiiro switches his smile from both, so they're both accurate

1
The smile resembles more to Kiiro's one but wider and having a more psycho look