I nominated the Caliban books. Donald and Prospero are both robots (as is Caliban). The author unhelpfully names many of the robot characters after Shakespeare! There's 3 books: Caliban, Inferno and Utopia. Prospero only appears in the second two but Donald (and Caliban, also a robot) are in all three. Some info here:
Caliban books
I nominated the Caliban books. Donald and Prospero are both robots (as is Caliban). The author unhelpfully names many of the robot characters after Shakespeare! There's 3 books: Caliban, Inferno and Utopia. Prospero only appears in the second two but Donald (and Caliban, also a robot) are in all three. Some info here:
https://en.wikipedia.org/wiki/Isaac_Asimov%27s_Inferno