When Gideon told Pelleamena and Priamhark about Harrow opening the tomb, she told them because she wanted to, for once, have the power over Harrow that Harrow had over her. But she hurt Harrow way, way more than she meant to when they killed themselves, and I think that’s the guilt Gideon has been beholden to for seven years. That’s the reason she’s never hit Harrowhark, because she’s already hurt her too badly to forgive.

And that guilt made it so they weren’t equals–until the pool scene. When Gideon points out that Harrow’s actions led to the deaths of the 4th house, Harrow says that she didn’t know what her choices would mean for them, and then she says,

“The Ninth House is deep in their blood debt and I am undone by the expense. I- I did not want to hurt you, Griddle!”

Does that phrasing seem familiar? What else is Harrowhark undone by?

I think the blood debt she’s talking about is not to the fourth house, I think she’s talking about the debt that she owes Gideon. She kept Gideon at arms length, even after the whole “I must no longer accept being a stranger to you”, and that’s what killed the 4th and, in turn, hurt Gideon with the guilt of their deaths.

When this comes to light in the pool scene, we understand finally that they owe each other in ways neither of them forgive themselves for. I think the pool scene is not only where Harrow confesses her family’s secret or her feelings to Gideon, it’s also where both of their shame is laid bare enough that they can see each other as equals.