|“||Possessing a dark atmosphere, this game allows you to explore the world inside a dream (thus its setting). There is no particular story or purpose. It is simply an exploring game.||”|
— Translation of Kikiyama describing Yume Nikki on their website.
While Kikiyama seemingly gave the permission to Project Yume Nikki, who created merchandise, a web manga and a light novel with different writers and artists, it was nevertheless stated by them that Kikiyama's thoughts about the game do not necessarily align with the ones of Project Yume Nikki. It is just an interpretation of the game by said writers and artists, which is also why manga and light novel strongly differ from each other.
According to the developers of YUMENIKKI –DREAM DIARY–, Kikiyama was directly and actively involved in the creation as a supervisor. All official annoucements related to -DREAM DIARY- refer to Kikiyama with "they" pronouns, which could mean that Kikiyama would like to keep their gender ambiguous and/or has a nonbinary gender identity or is a team of several people.
KIKIYAMAHP is Kikiyama's official website pertaining to Yume Nikki. It can be found here.
There is an e-mail address within the "readme" file of the game, which is hosted at their website. It is email@example.com.
However, although this address accepts mail, it is unlikely that Kikiyama continues to check it for mail. Furthermore, take note that Kikiyama may only know Japanese and e-mails that are written in English could be disregarded. Kikiyama has also been known to respond very infrequently and any response that is answered will be answered in a very short, straight-forward response. As with everyone, please be respectful when attempting to e-mail Kikiyama.