Mosaic was a pre-war unicorn who worked for the Ministry of Arcane Sciences.
Mosaic is the twin sister of Gestalt. Together they were the head researchers for the Ministry of Arcane Sciences, working closely with Twilight Sparkle. They were regularly left in charge of the Canterlot hub in the absence of Twilight. She and her sister notably finished one another's sentences, which Twilight believed was down to some form of telepathy between both of them.
Mosaic and Gestalt were at Maripony during The Last Day, they were working as the megaspells fell and were consumed by the entity that Trixie had become. Gestalt and Mosaic are directly responsible for the creation of the green alicorns which have their green coats. They also seem to be responsible for the telepathic abilities of the other Unity alicorns.
Mosaic was a green unicorn mare, and identical to her sister Gestalt.
Gestalt and Mosaic shared a unique ability to communicate telepathically with one another. This ability extended to their magic as well, allowing one to take over the others casting.
Twilight Sparkle was of the opinion that with the exception of Trixie and herself, the twins were the most capable spellcasters of all.
In a Ministry of Arcane Sciences report, a unicorn expressed discomfort in how the twins casually finished one another's sentences, and remarked they were not so much twins as one mare stuck in two bodies.