Billy could be developed as a more vulnerable character. Since they are homeless, his tough - guy exterior might start to crack. He could show his fear and insecurity to Max, which would make their relationship more complex. Max, on the other hand, could become more resourceful. She might start to learn how to barter and trade the little things they have to get what they need. For example, she could trade a handmade bracelet for a loaf of bread.
Billy's character can be developed by exploring his past. Maybe his homelessness brings back memories of his difficult family life before. He could become more empathetic towards others in the same situation. Max could be developed through her interactions with different people on the streets. She might meet an old woman who teaches her about herbal medicine, and she then uses this knowledge to help Billy when he gets sick. Also, Billy could teach Max self - defense, which not only shows his protective side but also develops their characters in terms of survival skills.
To develop Billy in the fanfic, we can show his growth in responsibility. As a homeless person, he has to take care of Max. So he starts to think more about the future and how to get them out of their situation. He could learn new skills like how to fish in a nearby river for food. Max can be developed as a symbol of hope. Despite their difficult situation, she always has a positive attitude. She could be the one who encourages Billy to keep going, and she might start writing in a journal about their experiences, which could also be a form of self - discovery for her.