Maybe there are copies of me on countless world lines. Maybe all their minds are connected, forming a single "me." That sounds wonderful, don't you think? Being in all times and in all places. A strong love for someone... a strong belief in something... a strong desire to convey something... If all those things go beyond time to create "you"... ...then that would be magnificent. So don't think of it as letting me die. Even if the world line changes, as long as you don't forget me, I'll always be there. ... I'm sure that from now on you're going to suffer. You'll be in a world where no one but you remembers me. You care more about your friends than anything else, so I think it's going to be painful for you. I'm so sorry. But that pain will mean a lot to me. Whenever you're sitting around in the lab, whenever you sip a drink, whenever you walk around town ... it doesn't have to be every time - one time out of a hundred is fine. All I ask is that you remember me ... because I'll be there. I'll always be there.
From episode 22.