Simon's character can be developed by exploring his backstory. If he has a troubled past, it can be revealed gradually as he gets closer to Bella. Also, his relationship with Bella can make him more empathetic or more guarded depending on how their interactions play out. For example, if Bella is initially suspicious of him, he might become more defensive in his behavior.
You can develop their characters through their shared experiences. Let's say they go on a journey together to find a cure for a new disease that is affecting both humans and supernatural beings in Forks. During this journey, Bella can become more courageous as she faces new challenges, and Simon can show his loyalty and determination to protect Bella and find the cure.