Akane would likely be very confused and worried. She might initially think Ranma is playing a trick on her, but as she realizes it's real amnesia, she would try to help him remember things. She could become extra protective of him. Genma, on the other hand, would probably be frustrated. He'd be worried about how this will affect Ranma's martial arts progress and his ability to uphold the family honor.
Ryoga would be quite conflicted. On one hand, he's always been in competition with Ranma. But seeing him in such a vulnerable state with amnesia, he might feel a strange sense of sympathy. He could even end up helping Ranma out of some tough situations, all while grumbling about how it's not really because he likes Ranma or anything. And Kodachi would be intrigued. She might see Ranma's amnesia as a chance to finally make him fall for her, using her acrobatic skills and charm to try and win him over.