When writing, focus on the interactions between Stricklander and your OC. Use dialogue to show their relationship. For instance, if your OC is initially afraid of Stricklander, their dialogue could be stilted at first. As the story progresses, the dialogue can change to show growth in their relationship. Also, describe the settings well to set the mood for their encounters.
Think about the plot. It could revolve around a mystery that Stricklander and your OC have to solve together. Or it could be a story of betrayal where the OC discovers Stricklander's true intentions. Develop the story arc, with a beginning that hooks the reader, a middle full of tension, and an ending that satisfies.
First, research Stricklander's character traits. Know his goals, his personality, and how he behaves in the original story. Then, create your OC with a clear personality and backstory. Make sure their traits contrast or complement Stricklander's. For example, if Stricklander is calculating, your OC could be impulsive.