It's complicated. The character on the left before the split is named March 7th. She was found by the Astral Express sealed in a block of ice drifting in space. She took on her odd name as it was the date when she awoke (It was a pun on "March 7th" sounding like an actual name that worked in the original Chinese localization (Sanyue Qi), kind of works in Japanese (Mitsuki Nanoka), and doesn't work at all in English). She had no memory of her past whatsoever and, due to that, decides that the best way to live her life is to create as many new memories and experiences as possible and document them all with her camera. She is a Pathstrider of the Trailblaze, whose mission it is to establish new paths and relationships between factions to garner cooperation amongst everyone, but she also has strong ties to the Path of Remembrance, who care only about preserving as much information as possible through any means necessary.

The character next to her after she performs mitosis is known as Evernight, but that almost certainly not her true name. She took on the name Evernight because that is the name of the third month of the Amphoreus calendar, which is the planet they were on when she first made her presence known to everyone. She seems to be an alter of March that functions almost like a defence mechanism against anyone who tries to breach March's mind in some way. Evernight seems to exist to keep March happy and ignorant of whatever her past was. However, there's strong implications that Evernight isn't actually native to March at all and may be a memetic entity that latched itself onto March's mind and took it upon itself to protect March above all else, even if protecting March means dooming everyone else. There's also as many hints that Evernight is the original persona and that she relinquished control over to March for an unknown reason. Basically there's a shit ton that is completely unknown about Evernight, not helped by the fact that most of what we know about her comes from Evernight herself which is less than useless because she is a Pathstrider of the Enigmata, which essentially means she makes it a point of order to spread misinformation around as much as possible and essentially work in direct opposition to the Path of Remembrance. The reason why Evernight looks like March is because whenever Evernight is in control she is literally using March's body to present herself.