Commander Warren is a minor character in The Handmaid's Tale. He is a Commander in the Republic of Gilead, to whom Janine, or Ofwarren, as she is known, is assigned to.
In the novel
Commander Warren is presumably an older man who supported Gilead's rise to power. For his loyalty, he became a Commander and he and his barren Wife were assigned a Handmaid. It is implied that Warren himself is infertile, and that his Wife and Ofwarren colluded to have Ofwarren sleep with her doctor to conceive and pass the child off as Warren's. It is unknown if Warren was in on this plan or suspected it. Ofwarren successfully conceived and gave birth to a daughter, Angela; unfortunately, the baby was born with defects and only lived for a few weeks.
In the TV series
Commander Warren's full name is Warren Putnam. In the series, he has a slightly larger role. He had an affair with Janine, claiming that he loved her and intended to run away with her and their daughter. However, this was apparently a lie to get her attention, as he does no such thing and lets Janine be reassigned, leaving her distraught and even more unstable than ever.