Another way is by exploring their family backgrounds. Peter lost his family on Earth and was then adopted by the Ravagers. Nebula was mistreated by Thanos. By comparing and contrasting their family situations, they can find common ground. They could talk about how they both long for a real family. This can lead to a deeper emotional connection between them. For instance, they might start to think of each other as family in a way.
In a fanfiction, they could be on a mission to stop a group of space pirates who are stealing valuable resources from different planets. Peter's charm and piloting skills combined with Nebula's brute force and strategic mind make them a formidable duo. During the adventure, they face many challenges that test their loyalty to each other and their own beliefs.
Peter Quill's origin story begins on Earth. He had a difficult childhood as his mother passed away when he was young. After being abducted by Ravagers, he was thrown into a world full of strange and diverse alien races. This experience shaped him into the adventurous and somewhat roguish Star - Lord we know. He learned to survive in the harsh and unforgiving space environment, using his wits and the few Earth - based items he had, like his Walkman, which also held sentimental value as it was related to his mother.
Peter Quill and Mantis aren't siblings in the comic world. There's no established connection of that kind. They have their own unique roles and arcs that don't involve being related by blood.