For Draco, you can focus on his past experiences as a Slytherin and how they've shaped his current state. His relationship with his family, especially his father, could play a role. For example, his memories in the pensive might be related to the times when he was pressured to follow in his father's dark footsteps. The repairman could be developed by giving him a backstory of being an outsider in the Muggle world, perhaps a bit of a loner. His curiosity about the strange object (the pensive) can be his driving force.
A plot could involve the repairman being hired to fix something in an old, wizard - friendly establishment where Draco has hidden his pensive. The repairman accidentally activates the pensive and is drawn into Draco's memories. Draco then has to enter the pensive to retrieve the repairman and in the process, they both learn valuable lessons about each other's worlds and themselves. As they journey through the memories, they form an unlikely bond that changes their perspectives on life.