From Covenant set visit by Joblo:
QuoteWalter is a synthetic minus the human traits that David has. He's straight forward and logical like Leonard Nimoy's Mr. Spock.
The Walter model was created by the company after the "too human" David model from Prometheus freaked them out.
Walter is a very efficient Butler/Bodyguard/Technician. He's solely there for the purpose of the ship and the crew. There are no complications in his programming like we've seen with Androids in other ALIEN films. He's like Bishop from ALIENS but with less human traits.
Walter can appreciate emotion but it doesn't come into his world of thinking.
David sees Walter as Spoiler
his kin, perhaps a student or a younger brother.