Actually I found the answer myself, after searching high and low on the net! It is Phoenix 2772.
…Godo is one such child brought up to be a cadet and nursed by the beautiful robot-maid Olga…
…After learning that Rena is to marry Rock, Godo had become stricken with misery and pushes away Olga's advances when she shows signs of love (and previously jealousy) for Godo. With the crew all killed one-by-one by the Phoenix and the secret of its weakness lost in Saruta's last words the Phoenix finally destroys Olga by burning her to death, and Godo finally surrenders. As he cradles Olga, realizing how selfish he had been towards his only friend in the world, the Phoenix is subdued by the power of love and thus boards the ship in the form of what appears the cross between an angel and a peacock.
Amazed at Godo's love of living creatures, the Phoenix, speaking with a female voice "herself", falls in love with Godo and grants his wish of reviving Olga on the condition he gives her anything she wants of him (not knowing that this involves his carnal love and the Phoenix possessing the body of Olga to obtain this)…