He might be confused at first. Percy has always been a bit of an outsider among the gods, but he may not expect Artemis to hate him. So he would try to figure out what he had done wrong.
On the other hand, Percy might be sad. He respects the gods and Artemis is an important figure. Her hatred could make him feel rejected and disappointed. He may try to find ways to redeem himself in her eyes, like performing heroic deeds that are related to her domain, such as protecting nature or her followers.
Percy could also be angry. He's a hero and has saved the world multiple times. He may feel that Artemis' hatred is unjust and he might be tempted to confront her about it, to demand an explanation for her animosity.
One possible reason could be a misunderstanding. Maybe there was some miscommunication or false information spread that made Artemis think Percy had done something unforgivable. For example, if Percy was wrongly accused of harming one of Artemis' beloved creatures or desecrating her sacred places.
Percy is initially shocked. He's used to Artemis being this strong, independent, and in - control goddess. When she loses her memory and her behavior changes, Percy is at a loss. He tries to be patient and understanding, like when she doesn't recognize him or acts in a more child - like way. He feels a sense of responsibility to protect her and figure out what's wrong.
She would probably be furious. Artemis has a strong sense of justice and protecting the vulnerable. So, she would likely immediately start to plan how to deal with those responsible for Percy's abuse.
She could also be angry. Given her power and status, she might feel slighted by Percy. She may take some form of revenge, not in a malicious way but perhaps by making Percy's life a bit more difficult during his quests. For example, she could send her hunters to interfere or withhold some information that could be useful to him.
He may also try to figure out what went wrong. Percy is a smart demigod, and he would likely not just accept the betrayal without trying to understand the cause. So, he might start investigating, talking to other characters who might have some insight, like the other demigods or even some minor gods who are friendly to him.
Initially, Percy is likely to be heartbroken. He thought he had a certain relationship with Artemis, whether it was friendship or respect. So when she betrays him for Orion, he may go into a state of self - doubt. He could wonder what he did wrong and start to isolate himself. But as time passes, he may gather his strength and decide to face Artemis and Orion head - on, to either regain Artemis's trust or show her the error of her ways.
In some fanfictions, Artemis might react with intense anger. She is a protector, especially of the innocent. So she could immediately seek justice for Percy, using her power and influence among the gods to find the perpetrator.
Artemis would be in shock initially. Given her status as a powerful and independent goddess, the idea of being forced into marriage is unthinkable. She would likely try to use her divine powers to break free from the situation. If she can't do it immediately, she might start to negotiate. She could offer other things in exchange for her freedom from this marriage. And if Percy shows any sign of taking advantage of the situation, she would not hesitate to put him in his place.
Percy might be devastated. He could become withdrawn and moody, shutting out his friends and allies. He might also be filled with anger and a sense of betrayal, which could lead him to make rash decisions.
At first, Percy might be confused about his connection to Artemis. But as time goes on, he starts to understand her values more. He may learn to respect her love for nature and the hunt. Their relationship could develop into a strong bond of mutual trust, where Percy becomes one of Artemis's most trusted allies in her various battles or quests.