I'm not sure who Aku specifically is. Aku could be a person with a great imagination who tells all kinds of stories, like adventure stories, fairy tales or even true - life stories.
King Baldwin could be any of the historical figures named Baldwin. There were several throughout history. Without more details from the 'king baldwin story', it's difficult to determine exactly which Baldwin it refers to.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The character of Aku could be developed in a 'pregnant Aku in Samurai Jack fanfiction' by exploring how he interacts with the world around him differently. He may become more reclusive as he doesn't want others to see his 'weak' state. Or he could become more aggressive in trying to protect his pregnancy, which would be a new aspect of his character. Also, his relationship with his own power might change. Maybe he has to find new ways to use his magic that don't harm the unborn, which would force him to grow and adapt in ways he hasn't before.
One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.
Shy Baldwin is indeed fictional. Usually, such characters are crafted to add elements of entertainment and imagination to the narrative. Their fictional nature allows for various plot developments and creative storytelling.
Greg is the main character in this story. Given the title, we can assume he has some connection to the watch falling through the cracks. He might be the owner of the watch, or he could be in a situation where he is responsible for the watch and the accident that befalls it. Without more of the story, it's hard to say exactly what kind of person he is, but he's clearly central to whatever events unfold regarding the watch.
One of the best Baldwin novels is 'Go Tell It on the Mountain'. It powerfully explores themes of race, religion, and identity in the context of a young African - American boy's life in Harlem.