Mata, Itsuka. (また、いつか。)

Overview
Mata, Itsuka. (また、いつか., Again, Sometime.) is a Yume Nikki fangame made by Mpiku, starring Katsuki and Shun.

Gameplay
The objective of this game is to explore and collect effects in Katsuki and Shun's dreams. Both characters use their bed to sleep and their desk to save. When awake, the telephone is used to switch characters. When in the Nexus, 5 is used to drop an effect like in Yume Nikki.

Plot
Katsuki and Shun's relationship is unclear. The title "Again, sometime" may imply they were separated once and hope to eventually reunite. Katsuki and Shun both refuse to leave their room/apartment and explore a dream world. Additionally, both have short black hair and are described as sentimental.

They could be brothers, judging by their appearance. They could be (childhood) friends separated by life circumstances; judging by the telephone being used to switch characters, they have a positive relationship.

These similarities could also be interpreted as Katsuki and Shun being the same person, either through different points in time or split personalities.