The evidence before the court is, incontrovertible. Eirin's collar has been growing higher.

Question is, what caused this phenomenon?

the whole costume style is changing, the sleeves are more baggy and over all it looks more cowl like, it wouldnt surprise me if she has a cape or ends up with one

This is actually a picture showing all the changes in the collar.

And... other aspects.

Evil Perverted Overlord Eirin?

The collar is a symbiotic parasite of some sort. Trade off is Eirin gets larger breasts.

I want to read this "scary reisen"