In Chapter 1, Luffy mentioned that he will want about 10 men in his crew. This can mean multiple things, he wants around 10 total people which 13 is still about 10, or that he wants 10 MEN and 3 woman don’t offset that number.
There are many many theories about why there will be 13 Straw Hats (Luffy and his 12 crew mates), so let’s start from the top:
1- THE FOUR PER SEA THEORY
So this is the most well known and one of my favorite theories about the 13 Straw Hats. Basically it breaks down the Straw Hats we have and finds a pattern between them and use this to theorize the next Straw Hats. Each one has a basic archetypes with other similarities.
1. The Chained Beast
All the first members from each sea were first seen chained up and known as beasts.
East Blue – Zoro was known as a “Demonic Beast” by Koby. Luffy saved him from Morgan’s prison.
Paradise – Chopper is a beast and was first seen tied to Kureha’s sled.
New World – Jinbe as a Fishman was deemed as a beast and Luffy freed him from Impel Down.
2. The Female
The female that joins during each sea. So far both are incredibly intelligent and we witnessed them lose a female close to them during a raid as well as were initially part of an enemy group. Fun thing both are initially met in a sitting position
East Blue – Nami was a member of Arlong’s crew and is immensely skilled in cartography and navigation. We saw Nami lose her adopted mother in a flashback and when first meeting the Straw Hats is in a sitting position on a roof of a bar.
Paradise – Robin was a member of Baroque Works and is a genius archaeologist. We see her lose her biological mother in a flashback and when first meeting the Straw Hats is sitting on a banister of the Merry.
New World – There are many theories. The big two would be Yamato and Carrot. Anyway Carrot was appointed by Inuarashi and Nekomamushi to become the new Ruler of Mokomo Dukedom while Yamato decided to stay in Wano to help protect it. Before leaving Wano, Luffy told Yamato that she was free to join his crew at any time in the future and that they would come looking for her when she did.
3. The Craftsman
In each sea, Luffy picks up a craftsman. Both are leaders of other groups, both are related to other pirates, and both have had a serious fight with Luffy. As well have they all been pariah and generally disliked in their homes. Finally, when each joined they joined with a ship and a flag.
East Blue – Usopp, the son of Yassop from the Red Haired Pirates, was a leader of the Usopp Pirates. He was regularly chased out of his town and when he joined Luffy he was mainly the generic craftsman of the group. Usopp and Luffy fought in Water 7.
Paradise – Franky, son of an unknown pirate, was a leader of the Franky Family and was also chased out of town. He became the Straw Hats shipwright and created the Thousand Sunny. Franky and Luffy also fought in Water 7.
New World – Also many theories for this, but I believe Bellamy fits best. He’s a craftsman that has a Vivre Card for Luffy, so he can return. He and Luffy had a serious fight in Mock Town and then again in Dressrosa. And when he arrives he’ll come with a new flag for the Straw Hats.
4. The Perverted Gentleman
They are a unique group that show their affection for people in an unhealthy way. And are known as royals.
East Blue – Sanji the third son of Vinsmoke. We all know about his nosebleed issues and his perversion.
Paradise – Brook the Soul King. He’s also a perverted but in a different way. Rather than a teenager, Brook is more akin to the perky old man.
New World – There are many theories here. Some say Momonosuke, others say Kinemon. In any case, this is a long-gone possibility.
As I said there are a lot of theories about these and it’s a cool theory. But let’s move on to the theory.