Diane Meade is a Police officer and colleague of Detective John Jones, the Martian Manhunter.
Diane was created as a Lois Lane type figure for J'onn J'onzz in the Silver Age. She, along with the later addition of Zook, made up the major supporting cast for Martian Manhunter during the John Jones period.
She disappeared from publishing for several decades after the John Jones persona was abandoned, and was updated in the Modern Age to have shared a similar history with J'onn as depicted in the original appearances, save for the time frame, since she appears to be in her mid thirties in her Modern Age appearances.
During her appearances in Harley Quin, it is mentioned she has a son but it is never mentioned what has become of the child's father.
Major Story Arcs
Diane and John have been business partners for some time, although it's unclear how long. Diane informs John that they need paying clients soon or they would have to close there detective agency. Right on cue a Skye Donner, comes into the office and hires Diane and John to spy on her boyfriend who she believes is being unfaithful with her. Skye goes on to explain that she believes her boyfriend is cheating on her with his wife. John is unsure about taking the case but Diane insists a case is a case.
John and Diane found themselves on a stake out at the boyfriend's home, and soon the discussing turns to Diane's son and how she is worried "the hellion" maybe too much for the new babysitter. John and Diane are about to share a moment but then the boyfriend comes home and gets in an argument with his wife. Diane stays in the car while John goes to investigate.
Later back at the office, Diane and John talk about what happened at the boyfriends house but Diane seems off to John. Then when she kisses him, John realizes Diane is being possessed by Harley Quinn. Harley tells John she has taken over Diane's body and that she can tell Diane has a thing for John. John reacts by pushing Harley's being out of Diane's body. This cause Diane to pass out.
Later when Diane comes to, she doesn't know that Harley used her body on several occasions, nor does she know that Harley has told John how she feels for him. Diane does know something has changed between her and John, but John isn't ready to tell her his true identity.