Shounen K

Gender: Male
Hair Color: Grey
Eye Color: Red

Boy K, is a mysterious young boy that, initially, only appears to Aoto.

Besides Aoto and otherworldly beings (i.e. gods/wizards), Undine has seen Boy K anytime he spoke to Aoto. Later in the story, Midori and Akane were able to see him after losing their loved ones.

Within the anime,

(Source: Divine Gate Anime Wiki)