Monthly Afternoon
"Mysterious Girlfriend X" is written and illustrated by Riichi Ueshiba. A one-shot chapter was first published in Kodansha's "seinen" manga magazine "Monthly Afternoon" in 2004. It was then serialized in the same magazine from March 25, 2006 to September 25, 2014. Kodansha collected its chapters in fourteen "tankōbon" volumes, released from August 23, 2006 to November 21, 2014.