Headcanon : Edgeworth and Franziska protected each other from Von Karma’s abuse as children.
You know, like five-year-old Franziska couldn’t finish her assignment in time, but when Manfred asks about it and she gets terrified, twelve-year-old Edgeworth covers it up for her and puts blame on himself. Edgeworth would do that. Considering how he saved Phoenix as a kid, and how protective he was to Kay in Investigations, I think he is a kind of guy who just can’t turn away from children in need. Franziska was a child who couldn’t protect herself, so Edgeworth had to be there for her while nobody wasn’t.
And Franziska started doing the same for him after she grew up. Like “Miles Edgeworth is my LITTLE BROTHER so he is the one who should be protected and guided by me! How dare he always tries to help me as if I’m a weak child? How impertinent!” So Edgeworth protected her when she was so little, but since she became a teenager, I think she tried to protect and support him instead. Like that scene on AAI when she convinced her father to let them continue their investigation. Of course she would rather die then admit whom she learned that behavior from.
This is very important, because I think it would’ve been really hard for Franziska to accept the fact that Manfred was a terrible father. She worshiped and adored him for her whole life. So even after Franziska learned that the way her father treated her was unhealthy, I think she would still try to defend him. She would blame everything on herself, thinking that she deserved everything she’d been through. It’s a common symptom in domestic abuse. Edgeworth praised Manfred as a “God of prosecution” EVEN WHEN he was being prosecuted under a false accusation by that man. Ugh.
But even when Franziska defending her father like “No, Papa wasn’t wrong! He only punished me because I asked for it!” she still couldn’t deny all those shits he had done to Edgeworth. Manfred murdered the boy’s father, took the boy as his student, only to frame him as a murderer and joyfully watch him die. And Franziska was always there to see everything; she knew better than anyone that her father NEVER loved that boy. She remembered how Edgeworth always protected her, and how she protected him too. Even if she could try justifying everything her father has done to her, she couldn’t possibly do the same to the horrid crimes that man had done to Edgeworth.
I think this would let Franziska finally break out from her misguided belief that Manfred was a great father and she was a shameful daughter for him. The baby step. Just like that Edgeworth finally realized the true face of Manfred when his found out who murdered his father; he could forgive his mentor for everything, even for prosecuting him as a murderer, but he couldn’t forgive the murder of his own father. This would be really important experience for both Franziska and Edgeworth to finally move on.