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Her official profile says, "She is strong-minded and tsundere; a charming mysterious girl, who is innocent and expressive. She's naive, but usually pretends to know everything.". If Lapis can be seen as "light" then Merli would be seen as "dark".
Her race's concept, fairies, is that they are special people who can convert “songs” into “power”, and can only be seen by a subset of the human population. Merli's power draws away the energy of living things; the opposite of Lapis' power.
A free iOS and Android app was produced for Merli, called " Good Night Merli " (おやすみメルリ). Much like the one for Lapis, it helps you fall asleep and comes with an alarm.
A second app was released, called ラピスと魔法のメロディ, which acts as a sequel to "Lapis and the Wonder labyrinth" (ラピスと不思議なラビリンス). While Merli makes a brief appearance in the first one, she appears to make a more prominent appearance in this.