Yamato in his past when chained by Kaido and locked up in the Sacred cave to starve was saved by three Samurai who referred to themselves by no name but upon closer inspection, it appears they may have been prominent figures in Wano Kuni. Kaido’s comment about them being powerful swordsman and his intent to recruit them indicates the status of those Samurai. At the time of Kaido’s attack on Wano Kuni, all the Daimyo of the regions were defeated and stripped of their power. Oden and his Samurai challenged the Beast Pirates but ultimately lost to Orochi’s schemes. Sukiyuki disappeared while Orochi was present in the Flower Capital and was presumably killed. Yasuie received a message from Oden and chose to protect the Kozuki family. After losing his position as Daimyo, Yasuie then chose to remain among the people to help them through the darkness engulfing Wano. As for the rest of the Daimyo, which total three, they all challenged Kaido and lost. What happened to those three Daimyo after they lost to Kaido? Answering that question establishes the possibility of who Yamato’s saviors really were.
Kaido has been defined as a creature with immense power that has given him the luxury of knowing no true equal. Oden surpassed his expectations but the schemes of Orochi robbed Kaido of his chance of a battle to test his limits. Life became uneventful to Kaido and in order to substantiate that lack, he lost himself in alcohol and sought chaos. In an attempt to manifest what he believes he desires, Kaido has recruited strong soldiers, bolstered his army and weaponry and entered into alliances that strengthened his position as a military power within the world. Such a path has led Kaido to establishing the methodology of breaking down an enemies will and bringing them to the point where they swear allegiance to him.
Kaido carried out such an act with Hawkins. He attempted to do that to Kid and Luffy. And as we have found out, he locked three Samurai in a cave with his son in order to physically, mentally and spiritually break them. All so they would have no choice but to submit to him if they wished to end their suffering. Once Kaido defeated the three Daimyo’s that challenged him, he would not have killed them but rather have locked them up in order to convert them over to his side as his soldiers. Yes, the three Samurai in the cave with Yamato were Shimotsuki Ushimaru (Daimyo of Ringo), and likely the Fugetsu Daimyo and the Uzuki Daimyo (leaders of Kibi and Udon and clans that have yet to feature within the Wano Kuni story).
The ones who entrusted their hopes to Yamato were none other than the Daimyo’s of Wano Kuni who could not save their people and nation. But in their darkness and lamentation, they found a path forward in Yamato’s survival and laid their very lives down upon such a path so that it could blossom into pillars supporting Yamato to the day twenty years from than. The Samurai fought on that day so that Yamato could survive to carry on fighting. They believed in Yamato and entrusted their wills to him. For twenty years Yamato has not been fighting for Wano by himself, he has inherited the wills of the Daimyo’s that wished for Wano Kuni’s liberation and Oden’s wishes. The Daimyo’s were all captivated by Oden and believed in his words and wishes. Wano Kuni needed to open their borders and the people needed to assist those that would arrive to fulfill the work Joy Boy left incomplete. The Daimyo’s understood the importance of Oden’s words after reading his journal. Yamato now carries that will. And the one who represents the person Oden was waiting for is Monkey D. Luffy. Yamato is fighting to open the borders of Wano Kuni and will help Luffy carry on Joy Boy’s will.
The story of three nameless Samurai escaping the Sacred cave is not over. There is still the story of them stealing the Inu Inu no Mi, Model: Okuchi no Makami Devil Fruit from Kaido and giving it to Yamato to eat. In order to survive, Yamato ate the Devil Fruit. And now, twenty years since that day, Yamato is using the power from that Devil Fruit to hold Kaido back so that Luffy has sufficient time to recover and return. Through Yamato, the efforts of the Daimyo’s manifest itself. Their spirit dances along with Yamato as he now battles his father.
When Yamato meets Zoro, he will recognise the similarities between that unnamed Samurai and Zoro. Yamato will come to learn the identities of all his saviors upon encountering the Samurai of Wano Kuni i.e. Hyogoro and the Akazaya Samurai. Oden was not wrong when he told Kaido that even if he dies, his spirit will live on. It now does through Kaido own son, Yamato.