Kathy Marshall has always been a character who kept her cards to her chest. We know already by this point, due to a comment made to Viv Martella that she was in a domestic violence marriage previously. This was also referenced a few episodes before in a comment to her GP.
She is called a to DV situation where it is all brought back. She has also spied her ex husband who is using tricky means to find her because he is in trouble.
Quinnin has upset her by an odd comment made in the canteen which she may have reacted to worse than usual due to spying her ex in the station. He immediately feels bad and wants to apologise, showing his development, not just in himself but also as a member of the relief.
A bit confrontation happens in the hospital where Quinnin gets involved in the situation and comes to Kathy's aid essentially building the bridge.
So hard to watch and yet this is one of my faves... I never get bored with it. Also a great calm before the storm moment between Viv and Kathy where they discuss her past. Viv makes a comment about people putting up with DV and immediately apologises for it but that she "will never understand it". A great way to show the difference between the characters.
Viv is also made aware that the DS who just caught her attention is actually Kathy's abusive ex. Cryer also smells a rat and is seen investigating the situation, the assumption is he knows about Kathy;'s background as they never actually make it clear who does and doesn't know. He wants her however a bit too late.
A lot of character development all round.... and Kathy's defiance against her ex at the end where she shows she has the upper hand (despite still being terrified) I imagine would inspire other DV victims all over the world.