Merli (メルリ / Meruri) is the winning name for the contest to find the name of the elder sister for Aoki Lapis. If Lapis can be considered as "light", Merli is therefore "dark" due to her more devil-like attributes. Although they're sisters, she and Lapis are enemies.
Her official profile says, "She's strong-minded and tsundere; she's a charming mysterious girl, who is innocent and changing the facial expression all the time. She's basically naive, but so innocent that she usually pretends to know everything ad hoc."