Theo

From Hoshikuzuu Scans:

In this country, there exists a god called the “batsu”. And a surreal and beautiful batsu called Rei is the master of a servant called Theo. Their roles were passed down from generations in their family, but their relationship gradually turns to love as Theo’s desire to stay together grow stronger day by day. When he feels the warmth of Rei’s body, he feels comfort. However, batsu age several times faster than normal humans, and Theo will also eventually outlive Rei.