Midou gets an additional makeup afficianado friend; Hiura gets another makeup coach. This is gonna be helpful for both of them, individually.
On top of that, both have someone new shaking up the dynamic of "makeup artist/makeup user" that is going to force them to come to terms with what their relationship actually means- Midou can justify getting closer to Hiura via makeup, and vice versa, but once the sancity of makeup is no longer just theirs, then they need to admit that the reason they matter so much to one another and want to spend time with one another isn't just that Midou gets to do makeup on Hiura and Hiura gets to cross dress- It's that they do, genuinely, love each other.
Gal is gonna be the ultimate wingwoman, whether she means to be, or not.