Ellie was the first person Haru met off his island, meeting in a casino after Plue had wandered away from him. The pair travel together after Haru rescues her from being attacked by a member of The Shadow Guard and as time goes on there are a few inclinations that they come to see each other as more than friends. When the new King appears and makes a move on Ellie, Haru is enraged by this and vows to make King pay for bringing tears to her eyes. Prior to the final battle, they confess their feelings for each other and agree to marry should they live through the war against King. I think this is a very subtle romance that plays out very well, since there's rarely any inverse inclination or hints that Haru and Ellie are in love until towards the very end.