Okay her name is weird, hear me out. Juniper and Starlight's fusion is Mirror Antithesis, which is rather redundant. 'Mirror' because, well, Starlight and Juniper met through a mirror. An 'antithesis' in fiction is a character who contrasts another, similar to how Starlight contrasts with Sunset (and Twilight, I guess. They're like a little antithesis triangle).
Moving on to her personality, Mirror is very perceptive girl. She sees what others miss and she is quick to point these details out. She enjoys directing movies (given the chance), and she's a good friend who has learned from her components' mistakes.
Bases belong to SelenaEde, PrettyCelestia, and AurBases (i think that's everybody)