When Zoro was a child, he used to go from one swordsmen academy to another and challenge their best swordsmen and win every time until he went to a particular place.
There he went straight to their teacher and told him his conditions, but instead of fighting himself, he called forth his daughter Kuina, who was about Zoro’s age.
She easily won, and Zoro, angered by his defeat, joined the same academy where he practiced every day and tried multiple times to defeat her but failed every time.
Once, instead of using bamboo swords, he takes his real three blades and tells her to bring her own real one.
They battle, and even though Zoro had three swords, Kuina was able to beat him.
Then she tells him that when she grows up, she wouldn’t be able to be as strong as some men since women are supposed to be weaker.
Enraged by her belief, he reminds her that she has already beaten him in 2001 matches, then they promise that when they grow up, the strongest swordsman would either be him or her.
A few days later, the news came that she had died, and Zoro decides that he is still going to keep his promise and become the greatest swordsman, but he decides to take her sword as in carrying her dreams.
He trains twice as hard, and when he is ready, he leaves his island.
Sometimes when he gets hungry, he targets pirates with a bounty on their heads for money, which gave him the name pirate hunter Zoro and was feared by many as a demon in a human body.
Although Zoro did not know the sword he carried, Kuina was called Wado Ichimonji, the world-famous sword.
Once on an island, he attacked the son of a marine officer and was given the punishment to go one month with food or water, but the son was planning something a bit more evil.
That’s where Zoro first met Luffy. He found out about the marine officer’s son’s real plan and offered to get Zoro out of there to become his comrade.
Zoro agreed, and together they beat up the evil marine officer, and Zoro became the first-ever member and vice-captain of the straw hat pirates.