ZoLu

"Luffy? Can you hear me? Sorry for worrying you. If I don't become the world's greatest swordsman, that would be a problem for you, wouldn't it? I...I will...I WILL NEVER LOSE AGAIN! Until I defeat him and become the greatest swordsman, I'll never be defeated again! Any problems with that, Pirate King?!"

- Zoro.

ZoLu is the slash ship between One Piece characters Roronoa Zoro and Monkey D. Luffy.

Romance Dawn Arc
Luffy meets Zoro while he's being held captives by some marines who agreed to set him free only if he survives with no food or water tied up under the sun for a month. Luffy admits to him that he's looking for a crew so he can become King of the pirates, to which Zoro replies that there's no way that he'd join his crew since he's 'weaker'. Luffy then says that he wouldn't want Zoro on his crew since he's got bad reputation. When zoro asks Luffy to feed him the rice that a kid prepared him which has been thrown on the ground and was filled with dirt, Luffy deducts that he's a good guy. When Luffy found out Zoro's going to be executed in 3 days and that Helmeppo lied, he proceeds to beat him up and declares that he's gonna make Zoro join his crew. As Luffy and Coby head back to the marine quarters, Zoro finds out what Luffy's done when he found out about his execution. Once Luffy gets his swords back and protects Zoro from gunshots, Zoro comes to admire Luffy's strength and tell him that he's earned a crewmate.

Orange Town Arc
After Luffy's been tricked into being captured by the Buggy pirates due to Nami, Zoro finally arrives to the island and looks for Luffy. When he arrives to set Zoro free, Buggy engages a fight with him and wounds him. Luffy tells Zoro to leave at which Zoro refuses, but then catches on Luffy's plan just through eye contact. After firing at the Buggy pirates, Zoro carries the cage Luffy's in and leaves. Zoro then goes to sleep and is put in care in a shelter by the mayor. After Luffy's cage is destroyed by an enemy, he tries to ignore the fight to go look for Zoro which failed and he ends up engaging in the fight. Buggy then fires a cannon ball at the shelter Zoro's staying in, which worries Luffy but turns out he came out alive. After Luffy attacks the buggy pirates recklessly, Zoro admits that he 'worried him for nothing'. When Buggy tries to intervene in Zoro's fight, Luffy stops him and says "Keep your mitts out of Zoro's duel!" After Zoro's victory, he admits to being a pirate and tells Luffy he's gonna rest. Luffy tells him not to worry and that he's gonna take it off from there.

Baratie Arc
Nami, Usopp and Zoro are all sat on a table when they call Luffy, who's been hired as a 'chore boy' because of the damage he caused. Zoro then teases Luffy about how he won't get to enjoy the food here which annoyed Luffy and made him drop a booger in his drink. Zoro pretends he's about to drink it then immediately makes Luffy drink the cup. Near the end, Zoro channels a fight with Mihawk, which in he immediately loses to. After being defeated, Zoro's ashamed and declared that he'll never lose again so he'll become the world's greatest swordsman and not disappoint him.

Arlong Park Arc
After Zoro beats the fishmen up, he wonders where Luffy is, saying that he promised he'd bring Nami back to him. When Nojiko's about to bring up Nami's past and Luffy says he's uninterested and leaves, Zoro said not to worry about him and that's just how he is.

Fan Fiction
Luffy/Zoro tag on AO3

Pixiv
Zoro/Luffy on Pixiv