It's a Japanese anime movie.
I watched this movie way back when. It was on a channel called Animax as a sci-fi marathon. This is what I roughly remember:
It's about this AI boy who has the memories/consciousness of a boy that passed away. The boy's parents put his consciousness in the AI so that they can have their son back. The doctor/scientist that is in charge of this boy has a daughter. The boy befriends this daughter/sister. After a while he sees that the girl tries to commit suicide by jumping off a roof. He asks her why and she says that she has brain tumor/cancer which makes her lose her memories of people. She shows him a gun and tells him "If I ever forget you, please kill me." (approximated phrasing). The girl eventually forgets him and the AI develops awareness instead of taking in the dead boys consciousness. The movie basically ends with the girl dying and the scientist telling the boy "It's like she is right here with us." And the AI saying, "how did he know." and they show that he has a copy of her consciousness too.
I have been trying to find this movies for like 10 years. Please anyone help me. Thank you.