Obviously, Luke Skywalker and Darth Vader are the main characters. Luke, the heroic Jedi, and Vader, the complex Sith Lord. In this fanfiction concept, their relationship is likely to be re - imagined in a very different way compared to the original Star Wars story.
In addition to Luke and Vader, there could be other characters from the Star Wars universe making appearances. For example, Yoda could show up in Luke's visions, guiding him on how to handle Vader in this new situation. Or Leia might be involved, either supporting Luke or having her own concerns about this strange new relationship between Luke and Vader.
Luke is the central figure as he is often the one who has to deal with the situation of having Vader as his 'little boy' in some sense. Whether it's a physical transformation of Vader, or a different timeline situation, Luke has to navigate their relationship. And Vader, even in this new context, still holds his power and mystery, being a crucial character.
One possible plot could be that Luke discovers he has a special connection with Vader that goes beyond their typical relationship in the Star Wars universe. Maybe he starts having dreams where Vader is trying to teach him a new form of the Force, something that was lost to the Jedi. And as Luke delves deeper into these dreams, he begins to understand Vader in a more empathetic way.
Yoda could also be an important character if the time travel somehow involves the time when he was still alive. He could offer his wisdom to Luke and perhaps even try to reach out to Vader's remaining light side, given his powerful connection to the Force.
Well, it could be a fanfiction where Luke and Vader have an unexpected relationship, perhaps a more tender or different take on their usual dynamic, like Vader showing a soft side towards Luke as if he were a little baby in terms of being protective or having some newfound paternal feelings. But without reading it, it's hard to say exactly.
Sometimes, new characters are introduced in these fanfictions that can be really well - written. They could be characters who act as mediators between Luke and Vader, or they could be side characters who are affected by Luke and Vader's interactions. For instance, a young Jedi apprentice who looks up to Luke and is terrified of Vader, but through the story, comes to see Vader in a different light.
Obviously, Darth Vader and Luke Skywalker are the main characters. Vader is the one on the hunt, and Luke is the target of his search.
The little angel could start off as a mysterious and seemingly innocent being. As the story progresses, it could be revealed that it has a much deeper knowledge of the Force and the history of the Skywalker family. For example, it might know secrets about Anakin Skywalker's fall that no one else does.
You could try searching on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in the title 'luke vader my little baby' in their search bars and see if it pops up.
One idea could be that Luke discovers a young Force - sensitive 'little angel' figure who has a connection to Vader's past. Maybe this little angel was a symbol of hope on Vader's home planet long ago. Luke then tries to protect the little angel from the remnants of the Empire who want to use its power for evil.
One common theme is redemption. Vader's journey towards redemption through his connection with Luke is often explored. It shows how love for his son can make him turn away from the dark side.
One common theme is mentorship. Vader could be seen as guiding the little boy in the ways of the Force or in general life in the Star Wars universe. Another theme is redemption. The presence of the little boy might inspire Vader to seek a path of redemption from his dark deeds.