In Chapter 983, Yamato appears first time and saving Luffy from Ulti and runaway with him.
Yamato seems to dislike being close to his father and his father’s crew, having disappeared during the Festival of Fire festivities. Rather, he is very interested in Monkey D. Luffy as he actively worked to get to know him after waiting for him to come to Onigashima. Luffy felt that Yamato had no intention of hurting him, which Yamato quickly stated upon meeting him.
Obviously, Yamato will be an ally now, and I think there is credibility in the belief that he has been waiting for Luffy to help him escape the island. It might even have some potential to join the Straw Hat Pirates team.