Agnes is portrayed as the loyal worker and a sort of old friend to David, because she's been there apparently since the agency opened its doors. She's polite and upbeat almost to a fault, but she's not afraid to put her foot down when necessary. You can only push her just so far; when Agnes thinks you're not doing right, you'll know about it. She hands David his hat in scathing monologues in Brother, Can You Spare a Blonde and Father Knows Last, to name two off the top of my head.
I was wondering if she ever does this with Maddie. I think at the beginning of Poltergeist... and North by North Dipesto, she talks about how she doesn't seem to be appreciated around here, things like that, but I can't seem to remember if she ever really opened the cannons against Ms. Hayes the way she did with David. Maybe she thinks David bears more watching because he's been around longer. Or it could be she feels sorry for Maddie, who shows up as the victim of a serious crime, is something of a loner and must learn to get along in what is for her an entirely new environment. Despite being younger, David is much more worldly in his way and maybe Agnes is trying to help Ms. Hayes a little more, since she doesn't know the ropes.
Still, I wondered if Agnes ever had to bring down the hand of justice with Maddie the way she does with Dave. None come to mind, but that doesn't mean they couldn't be out there.