Rin accidentally saves Kouki Maeno in Episode One, and realizes he has a distorted memory. After discovering the truth about his past, he chooses to keep on living and began working at Asogi Consulting, where he spends the next twenty-one years.
Kouki is the only "normal" member of Asogi Consulting, and had to do a lot of growing up to function alongside Rin and Mimi, since he was a little squeamish and wasn't accustomed to the copious amounts of vodka Rin and Mimi often drink.
By the end of Episode One, we learn that Kouki is a clone created by Aoyama Pharmaceutical Company by Sayara. The original Kouki died during an experiment that involved his vivisection. His memories lacked realism, as it is actually the memory of the original Kouki written into the clone's brain by artificial means.
Sometime between Episode Two and Episode Three, he marries Yuki and has a son named Teruki with her. In Episode Three, he is fatally shot by Sayara while rescuing Rin off Hagashima Island, and had to consume Shogo's Time Fruit in order to survive as an angel. After saving Rin and having one last tender moment with her, he returns to consume Sayara, but is killed (unlike that of immortals, the death of an angel is permanent regardless the time fruit) by Apos before he can finish.
Kouki's time fruit is lost in the sea, but it is inexplicably found by Rin while she regenerates in the ocean between Episode Four and Episode Five. His granddaughter Mishio acquires it after Rin's body is destroyed in episode five, unwittingly brings it to Apos' castle in Episode Six, and passes it to Rin as the latter is consumed by Yggdrasil. Kouki's spirit then helps Rin to break free.