Hotori: Tada Saiwai wo Koinegau

At the Personality Plant, robots are being built and slowly outfitted with the artificial memories of real people. Suzu is one such robot, being made to replace the young boy a family has lost. By chance, he meets a Hotori, a young girl suffering from progressive memory loss. The two children become friends and attempt to define themselves in the light of their changing memories.
(Source: Anime News Network)