To become the king of someone else's story, you need to be unique and memorable. Do things that no one else does for them. For example, surprise them with small but meaningful gestures. Listen attentively when they talk, and offer advice or just a shoulder to cry on when they are down. Build a strong emotional connection with them. Be the person they can't imagine their story without. This means being consistent in your actions and always bringing positive energy into their lives.
One way is to be a great storyteller yourself. Share amazing experiences and insights that make the other person want to include you as a significant part of their own story. Also, showing genuine care and support for their dreams and goals can help. When you actively contribute to their growth and development, you are likely to be seen as a king in their story.