I think a lot of people don't quite gets Yona's arc. She's lost everything, family, tribe and at one point she believed her life. But more importantly, she believed she failed to avenge her father and her ancestors. That alone has caused her to withdraw into herself. She has no drive, nothing left in the tank and it shows in her early scenes in the house. Yona's quite willing to follow everyone else's lead in anything.
Its only after the strings around the house are starting to snap that Yona is starting to realise she has a second chance, and fuck anyone who gets in her way, she's going to take it. Of the character archetypes, Yona's the fiery one, the one with the short temper, but all that has been subdued since she arrived at the house. But what we're seeing now is someone is throwing fuel onto that fire and I will not be surprised if by the end of the arc, Yona is the fiery amazon she was presented as. Goodbye to the MC's hips, that's all I'm saying.