I know the tags and artist's intentions claim it's just Lumine with an alternate pink hair color, but honestly, I thought this was Yae cosplaying.

Though Yae sometimes likes to tease that she will never reveal her fox form, I could see her shapeshifting to look like Lumine to surprise and fool Aether (or vice versa), simply to make a point and uphold her race's reputation as master tricksters. Just for fun, of course.