Here, I translated the Hikaru Utada's interview into English (Please forgive my poor English).
Utada： “About KOHH, my friend taught me a while ago and I became a fan.
At the beginning, this song was intended as just an instrumental. When the suggestion that how to put a lap came out, I(=Utada) told director that "There is a person called KOHH." Then he (=the director) answered "Oh that is nice!” After approaching KOHH, it turned out that we were actually fans of each other. He decided to join us.
First I made my part and told him about my ideas about "oblivion（＝忘却、the title of this song）" and "memory". He responded with my own words with spending several days.
It was my first time to mix my words with other people, so I thought “What would it be like?” But we naturally met in the middle of us.
I like a long rap part, so I left the first one minute to him. When we got lost, I told him "this way", and we were together checking with a feeling like "It's okay". As the last organ was his proposal, it became an image like I was called to heaven. So did it become a requiem?
To think about a way of living is like to think about death. I looked back on my life and thought about the way I am heading from now. Everything is condensed into the last line, "When I someday die, empty-handed would be best ".”
KOHH："No comment. First of all, please listen to the song."
↓Here, you can find the original version of her interview in Japanese.