Shimotsuki Kuina, Roronoa Zoro's childhood friend in the One Piece series, shapes a lot about the character of this swordsman and his aspirations. Kuina lived in Shimotsuki Village, the same village where Zoro had grown up. She was the daughter of the master of the local dojo and, a few years his senior, was a prodigious swordswoman who always managed to win against Zoro in their many duels.
Kuina and Zoro shared a fierce rivalry that blossomed into mutual respect. Interactions between the two of them laid the foundation for the development of Zoro, both in skill and will to be the greatest swordsman in the world. This bond was very strong because of the common passion toward the way of the sword and the competitive spirit between them. Through this relationship, the foundation was set on which Zoro would firmly adhere to his will in the way and seek strength without relenting.
One of the most significant events in Zoro's backstory involves Kuina's untimely death. According to the canon stated in the series, Kuina died from falling down a flight of stairs. This tragedy occurred while Kuina and Zoro were still little kids. According to legend, this incident changed the course of young Zoro's life and his aspirations. The very suddenness of Kuina's death and the seeming banality of its nature have led her death to be highly debated by fans, some even going so far as to question the circumstances behind her death.
The eve of the day she died, Kuina shared with Zoro a moment that defined Zoro's future: Kuina getting frustrated over people saying that women couldn't be the strongest swordsmen in the world. She challenged him to a duel with real swords, and he lost. After that defeat, they promised each other that one of them would be the greatest swordsman in the world. This promise became what drove Zoro's ambitions after Kuina's death.
Upon Kuina's death, Zoro inherited his longsword, Wado Ichimonji, and took it as one of his three primary swords. This constantly reminds him of the promise to Kuina, of the dream to become the best swordsman in the world. The fact that he continued to use the same sword throughout his journey symbolizes the impact Kuina really had on his life and wishes.
Kuina's influence does not stop at his influence on the motivations of Zoro; one can also see the effect of this in terms of the interactions he shares with other characters throughout the series. Most notably, Zoro's interactions with Tashigi, a Marine officer, have been shaped by Tashigi's striking resemblance to Kuina. This resemblance often catches Zoro off guard and brings out the memories of his childhood friend and rival. In fact, this similarity has caused some fans to take their idea a step further by speculating on a deeper connection between the two characters.
Kuina also had an impact on Zoro's fighting style. Indeed, part of his Santoryu technique was originally developed due to the fact that he could not defeat Kuina with normal methods. This progressive concept of swordsmanship became the signature style of Zoro and a major part of his character as a warrior.
These glimpses of memories regarding Kuina come throughout the series at specific times and are often used by Zoro to either inspire or reflect on himself. In especially tough battles, it is frequent for Zoro to recall his promise to Kuina and draw strength and determination from that promise to win where the odds might otherwise be against him.
Although most of Zoro's background is not touched upon in the main storyline of One Piece, its effect on his relationship with Kuina obviously shows in what he does and how he comes across. Losing Kuina early in his childhood set Zoro to become a resolute and focused swordsman he is primarily tagged as being across the series. Her memory serves both as a driving force and as a touching reminder of promises and dreams fueling up Zoro's journey.