Chaos;Gate

Chaos;Gate (also known as ChäoS;Gate) is a Steins;Gate and Chaos;Head crossover light novel that takes place one year before the events of Steins;Gate. It details a chance encounter between Sena Aoi (Chaos;Head) and Rintaro Okabe in Shibuya, Japan. The story was written by Naotaka Hayashi (Steins;Gate, Chaos;Head visual novels' main writer), and was initially published in the Nitroplus Complete magazine released on September 30, 2009.

Synopsis
While chasing a porter carrying a N.O.Z.O.M.I.Technology terminal, Sena Aoi encounters a high school-aged Okabe within the Shibuya Station. When Sena's calm and composed personality clashes with Okabe's overtly pompous antics, sparks will fly...

Trivia

 * Chaos;Gate was published 15 days prior to the initial release of the Steins;Gate visual novel.