
She was very happy with her life depending on her brother, but now that he is gone she is starting to think. That last panel (Chapter 5) was incredible. It’s like she suddenly realized: “Ah, so even you brother think I am weak and useless.” God, I am expecting some good character development!

I have the feeling that she's shocked because her brother is ok with him to sacrifice his own life for her. And not ask her for rescuing him or anything.
She has been crying for him all along, and even willing to exchange her heart for him, clearly, she loves her brother.
Even before her brother's message, she was pondering over the "fraudulent" contract with the swords, and worries about her appearance.
I think his message will solidify her willingness to save him, but not because she's mad. The main reason would be he's the only one who thinks of her well-being, instead of shoving tons of responsibility on her and judging her, fully aware that she has never been prepared for it.
Anyone has the RAWS?
https://newtoki95.com/webtoon/25309?stx=%EB%8B%A5%ED%84%B0+%EC%B5%9C%ED%83%9C%EC%88%98&title=%EB%8B%A5%ED%84%B0+%EC%B5%9C%ED%83%9C%EC%88%98