Leo chose to become a dragon. After all, being one unleashed your full potential and strength which she pursues, in addition to being called weak by her partner.
Just imagining Ragna killed the current king, then Leo steps up to become the new king and in a bittersweet meeting they fight each other as a rematch. Man, I would hate it if this manga turns out that way, the typical dramatic twist that makes things super hard to move through.