I just finished reading this webnovel last week, so this is fresh. The title is often translated as 'Cavier Falcon Princess' but you'll also find it as 'Poeun's Daughter' or 'Poeun-ui Ttal'. In terms of major character deaths, the one that absolutely wrecked me was Jihu's. He's the Crown Prince's bodyguard, the stoic, loyal one who has been by Lady Lee Soo-yeon's side through everything. His sacrifice near the end of the second major arc—protecting the princess during the palace coup—is brutal and beautifully written. It's not a sudden shock; the tension builds for chapters, and when it happens, it's all about duty and unspoken feelings.
There's also Minister Park, the older advisor who is a genuine ally to the princess. He gets poisoned. It's a quieter death, but it really shifts the political landscape and forces the princess to step up without his guidance. I've seen some readers online debate whether a certain scheming concubine dies, but she actually survives, exiled to a convent. The deaths in this story feel earned, not just for shock value, which I appreciate.