MEZZO" is a 2-person sub-unit consisting of Yotsuba Tamaki and Osaka Sougo from IDOLiSH7.
During the joint practice portion of the Main Story between IDOLiSH7 and TRIGGER (around Part 2 - Chapter 9) Tsunashi Ryuunosuke from TRIGGER is added to the group for this period.
MEZZO"'s unit name comes from their respective musical symbols, Mezzo Piano (Yotsuba Tamaki) and Mezzo Forte (Osaka Sougo).