This appeared in the D.C. II anime, and is a very sad, yet beautiful song. The song title literally means “Don’t Disappear”.
Will modify this post with the translation soon.
Some lines: “Here I am, right here / Hold my hand and never let go. / Hey, don’t disappear, I don’t want to forget you / Even when I am here because you are here / Hey, don’t disappear and leave me alone / I am thinking of you with all my heart”