Nana Saotome

She first appeared in volume 3 of the manga and Episode 12 in the anime. The strange coded message that Ritsuka receives from SLEEPLESS is from her. Ritsuka meets her in a special area of an MMORPG called Wisdom Resurrection several times to discuss The Septimal Moon and Seimei. Her mission as stated in volume 5 is to determine whether Seimei really faked his death. She is a member of The Septimal Moon, although her role in the organization (outside of the previously described mission) is unclear. She has tell-tale signs of a hikikomori, leaving her computer rarely and has even admitted to Nagisa she has no other friends. Her whole name is given in volume 9.

(Source: StateMaster)