credits to user Sahlin for the spoilers in novel forum. There are side stories but she/he said that some of the side stories are scrambled up but here are the gist of the important parts.
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Valhail asked her when she fell in love with him, Zelle replied that it was when he took her to see the crystal roses, so Valhail later proposed to her surrounded by crystal roses. They got married and got a son really quickly. First prince is Russellzen (Regen). Second prince, four years younger, is Kargale. Third princess, ten years younger than Russellzen, is Artiel. Artiel is doted on the most. When Russellzen was 15, the young emperor Recalius of the Belott empire (age 8) greeted them and must have made a mistake, because Artiel scolded him loudly. Amusingly, Recalius seemed to fell in love with Artiel, which made Russellzen and Kargale really protective. Recalius later start learning martial art from Valhail.
Princess Zelle was the eighth princess of Roymond, who reduced her presence so she won't be seen as a threat killed by her older siblings. Because of that, she avoided everyone of power and kept to herself. When Granor invaded her kingdom, she and her younger siblings fled to distract the enemies from her older siblings, the more powerful royal heirs. She was captured and woke up as a falcon.
She floundered around before becoming a messenger falcon of Valhail, the commander of the Roymond army and her 2nd sister's fiance. Because normal cavier falcon aren't smart, they keep getting captured by enemies. Valhail noted that Zelle seemed to be able to understand human speech and was initially suspicious, thinking that she is a spy. He started trusting her when she was injured on her first mission because the enemy wouldn't hurt their own spy. Zelle was a bit reckless because she prioritize the success of the army over her own life, which made Valhail mad a lot. She managed to figure out the map of the enemy's camp and was so well successful that the Granor called her the 'Eye of the Sky'.
Later, the Granor army claimed to have Zelle's body and threatened to kill her if the Roymond army attacks them. As the only known royal heir alive, Zelle is very precious to the Roymond army. Valhail managed to start a surprise attack, win the battle, and reclaim her body alive. However, they found that she was poisoned and would die in 30 days without an antidote. Bird Zelle got frustrated and bit her body on the neck, trying to wake herself up. Then Zelle flew off and was shot by poisoned darts after getting lured into a trap.
Zelle woke up as human again. Valhail started suspecting her to be a spy after she slipped a few times (she was supposed to in a coma ever since she was captured so she couldn't be aware of what's going on in the war). Eventually, it boiled to a point where Zelle admitted she was the bird and start quoting back the things Valhail said to the bird. Valhail was stunned, and while coming to terms with the truth, Zelle escaped. She was attacked by barbarians but was saved by Valhail, who admitted to believing her.
Spoiler: their reaction
Zelle used her presence to boost the army's morale (the king and all his concubines are dead, along with many heirs). They continue to search for the remaining heirs (1st, 2nd, 4th, 9th, 10th). At one battle, they tried to divide the much larger Granor army. Zelle offered to be a bait, but was turned down by Valhail in favor of a fake bait. The battle was successful and the Granor retreated after taking a big loss. However, a Roymond assassin attacked Zelle, saying only the Eastern King should be the real ruler. Zelle suspected it was one of her siblings. She later strike up a friendship and alliance with Hilde, a politician from the East.
Spoiler: first kiss
Later, 4th prince was revealed to be alive. He claimed to have fought his way out of where he was imprisoned. Zelle was happy to see him until he later accused her of getting a good reputation and respect from the army by sleeping with Valhail and other higherups.
Spoiler: 4th Prince
Valhail got really mad at 4th prince for insulting Zelle. During an argument in the Armory, Zelle scratch herself on a spear with a poisoned tip and fell unconscious. She woke up in her bird body, which had been in a coma due to the same poison. Valhail realized this when the bird showed up and he worried about her a lot, refusing to send her to the enemy's camp. (One of his soldier even asked him if he fell in love with a bird because he was seen treating bird Zelle so tenderly. I think it was at this time that Valhail first admitted to one of his soldier that he loves Zelle) Zelle met Adele, a lady from the west who was thought to be a nerd. Zelle discovered that Adele is a genius and is well educated with eastern history. Zelle gave her some messages she got from the Granor and Adele managed to decode them, which talked about how the Granor are worried about their food supply. Valhail learned of Adele's intelligence through Zelle and asked Adele to identify the poison and figure out a cure.
Adele found that the cure is the poison after being diluted. They awoken soldiers that were poisoned in battle, but Zelle wouldn't wake up. Valhail later took Zelle to the cliffs where crystal roses grew. He told her that she should live her life however she want and he would support her. (Even during the war, Zelle didn't want to throw her presence around too much because she was afraid that her older sisters might kill her after the war is over and Valhail realized that) Zelle realized that she like him. Later, Zelle, in her bird body, participated in the war by gathering a bunch of birds and targeting the enemies' layout (they just followed her for some reason, which surprised her and she used it to her advantage) Adele figured out that both blood and poison were involved. She also figured out the history of Zelle's ancestor, who was said to have golden blood and shared that blood with cavier falcon and Zelle realize that could be the answer to why she can wake up in a cavier falcon's body. Zelle woke up after the new cure was administered.
After Adele met Zelle in her human body (she doesn't know Zelle is the bird, only Valhail and two of her maids, including the one that was stabbed, knows), she praised Zelle a lot because she found out through her investigation that Zelle faked a letter pretending to be 1st princess to save grain harvest from a typhoon and saved a lot of people from starvation a few years ago. Zelle was embarrassed because forging letter is a crime and she was trying to keep a low profile to stay alive. Later, rumors of how Zelle saved the grain supply spread though the people of Roymond. The Roymond army won the battle and reclaim Roymond's castle from the Granor. Zelle threw a banquet to celebrate. Granor sent a message asking to end the war.
Later, they found out the rumors about the 4th prince was true and Zelle thought that she must stop him from taking the throne. At the banquet, Valhail's people came and reveal that 4th prince did not fought his way to freedom, but his mother bought him with the lives of 200 Roymond people. A lot of people were angry. Valhail fought with 4th prince and mocked him for his perverse taste toward women who resembles his sisters, humiliating 4th prince in the battle to avenge the victims.
Not long after, 4th prince was found dead, having been killed brutally. In addition, her 2nd sister showed up now that the war has ended with soldiers of her own. She admitted to starting the war with Granor to get rid of 1st princess, but Granor betrayed her. She was the one who was bought out of prison with the lives of 200 Roymond people, but framed the shame on 4th prince. She was the true Eastern King (Her initials were the anagram for Eastern King. Zelle thought it was either 4th, 9th, or 10th because their maternal family are from the East) Zelle tried to run away with Valhail but surrendered after 2nd Princess threatened to kill Zelle's maids. She then imprisoned Zelle, trying to marry her off to Adele's brother, who was on 2nd Princess's side. But to do that, 2nd Princess must ascend the throne to gain the authority for that.
One of Zelle's maid slipped her the poison to putting her soul in the bird's body. Zelle took it, trying to help Valhail find evidence of 2nd Princess's crime. 2nd Princess killed off two of her own men who could be important witnesses, disheartening Zelle for failing. 2nd Princess was also annoyed with Valhail because he was her fiance but he asked to broke off his engagement and even fell in love with Zelle. Zelle pricked herself with a poisoned spear, waking up as a human again. At the coronation, Valhail's men broke in with evidence of 2nd Princess's crimes and the witness is - surprise - Adele's brother who 2nd princess is trying to marry Zelle to. (Earlier, Zelle accused him of wanting to possess her more than actually loving her, or he wouldn't have let 2nd princess imprison her) Most of the soldiers did not know of 2nd Princess' treason and served her because she was now first in line to the throne with the death of 1st Princess. Valhail's private army is loyal only to Zelle because Valhail swore an oath of loyalty to Zelle during the war and his army only obey the owner of the necklace, which Valhail gave to Zelle. :blobnosebleed:
The 2nd Princess is carried away, screaming at Zelle the whole time, blaming her for everything. With no other heirs, Zelle took the throne and became Empress. She went to look for her bird body, hoping to cure the poison for good and find a way for the bird to live its own life (which might be impossible because it only contain Zelle's soul. When Zelle is in her human body, the bird is unconscious), only to find that the bird had died not too long ago, after Zelle was crowned Valhail said that the war was too harsh on the bird's body in addition to the poisoning. They buried the body and erected a monument at its burial.
https://forum.novelupdates.com/threads/cavier-falcon-princess-%EC%A0%84%EB%A0%B9%EC%83%88-%EC%99%95%EB%85%80%EB%8B%98.123096/