In Bridge to Terabithia fanfiction, Jess and May Belle often have a sweet and complex relationship. Jess, being older, might be protective of May Belle. He could take her under his wing and share some of the magic of Terabithia with her in the fanfic world. Maybe he teaches her how to be brave like he learned from Leslie.
One possible storyline could be that May Belle discovers Jess's secret place in Terabithia and wants to join him. At first, Jess is hesitant but then realizes it could be fun to have her around. Another could be that May Belle gets into trouble at school and Jess has to come to her rescue, strengthening their bond.