Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
His best friend Pikachu might be a bit confused at first but would quickly adapt as it still senses Ash's essence. Brock would probably be very surprised and might offer some comical comments or advice on how to handle the new situation. Misty could be a mix of shocked and intrigued. She might be a bit jealous of Ash's new female appearance at first, but then would support him as they are friends.
His friends would likely have a mix of reactions. Some, like Pikachu, would be right by his side and excited for the new adventure. Others, like his human friends, might be a bit sad at first because he'd be spending more time with the Elite Four duties. But overall, they'd be proud of him. They know how hard he's worked to get to this point in his Pokémon training journey. And they'd also be looking forward to seeing how he'd change the Elite Four with his unique style.
In some fanfictions, Ash gets angry. He might be angry at the world, at fate, for taking his mom away. This anger could manifest in different ways, like being more reckless in battles or snapping at his friends when they try to comfort him. But deep down, it's just his way of dealing with the overwhelming pain of losing his mother.
His other Pokémon like Rowlet might be in awe. Rowlet is a curious Pokémon, and having a mythical Pokémon like Tapu Koko around would be fascinating. It could learn a lot from Tapu Koko, especially in terms of using its unique powers more effectively. The whole team would likely feel a sense of pride that Ash was able to catch such a rare and powerful Pokémon.
Serena could be a bit shy at first. She might blush deeply and look down for a moment. But then, she would look into Ash's eyes with love and admiration. She would tell Ash how much she loves him and how she has been hoping for this moment. She would accept his proposal gracefully, and they would share a tender embrace while their Pokémon friends cheer around them.
His other Pokémon like Bulbasaur and Charmander would also be startled. They might gather around Ash - Zoroark and sniff him out to make sure it's still him. They could be a bit hesitant to follow his commands at first because they're not used to his new form. However, as they see that Ash's spirit is still there, they would gradually start to trust and follow him again, and they would work together to figure out how to turn him back.
Ash would likely be extremely heartbroken. He has always had a deep connection with Pokemon and believing that Arceus, a powerful and supposedly benevolent being, has betrayed him would be a huge blow. He might become withdrawn at first, not knowing who to trust anymore.