Plot
Adult character finds a child (13?) in the forest. Takes the child under their wing. The child is the reincarnation of this legendary person (most likely a warrior) and kind of acts a bit off. The adult is then tasked with finding out the mysteries of this child. Including their background, family, etc. The kid has an ancient power connected to them, yet they don't know it. (Dragon bonded, Plus other powers?) They adventure through trying to find clues.
Part 2: Main arc two is a guard/assassin. When the adult and kid visit the Kingdom where arc two is from, they are arrested and held in a cell at random. This triggers an event, that reveals a slight detail about the child.
Child's Backstory: I imagine they're a human in both lives, and the reason why they're so notable in their first life was because of that. In a fantasy world full of supernatural creatures, whose natural abilities and magic capability typically have a tendency to overshadow humans, not on purpose in most cases, it's rare for a human to make it into the legendary status. Especially as a warrior, and the dragon may have contributed to the image, but the character had been fighting for a while before they bonded to a dragon.