It has been almost 10 years since Ponyo moved in with Sosuke. It's her 15th birthday today! Fujimoto suggests to marry Sosuke, but this just seems to make their happy little lives into the storm that brought them together.
In the movie, it is clearly revealed that Fujimoto was ones human, but gave it up to go life in the ocean so... who was he before? This is my interpretation about who Fujimoto was, how he met Granmammare, how he came to dislike humans that much, and finally decided to stop being a human himself to go life in the ocean together with Granmammare.