Rollan and Meilin are both fictional characters of Spirit Animals series. They are known to have romantic feelings for each other and care for the other deeply. The word "Reilin" is the ship (a romantic pairing between two characters) between Rollan and Meilin.
evidence from the books below:
When Rollan met Meilin at Sunset Tower he described her as striking and when she gave a shy smile at him he stopped in his tracks. Later while tracking Arax the Ram and Rollan got stuck by a hail in the head and was bleeding Meilin released Jhi to help him. Later during the battle for The Granite Ram when Shane attacked Meilin Rollan tackled Shane to the ground.