One possible plot could be that after years of adventures and growing closer as friends, Percy and Reyna start to realize they have deeper feelings for each other. They might face some initial resistance from their friends who are used to them just being comrades. But as they work together on a new mission, their bond strengthens and they finally decide to get married. The wedding could be a grand affair with all their demigod friends in attendance, celebrating their union.
In a 'Percy and Reyna get married' fanfiction, it could start with Percy being lost and Reyna finding him. They end up on an adventure together, during which they rely on each other for survival. As they face various challenges, they realize they are perfect for each other. Percy's loyalty and Reyna's leadership skills complement each other. When they return, they announce their intention to get married. Their friends are initially shocked but soon come to support them wholeheartedly.
Well, perhaps in the fanfiction, Percy and Reyna are forced to get married for political reasons within the demigod world. At first, they are reluctant, but as they spend time together, they discover they actually like each other a great deal. Percy shows Reyna his more vulnerable side, and Reyna helps Percy understand the more complex aspects of Roman demigod culture. Eventually, they fall in love and their marriage becomes a real union of love, not just a political arrangement.