Jinbe

Jinbe is a fish-man, a Warlord of the Sea (former), and one of the strongest characters Luffy ever recruited. He joined the Straw Hats at 46 years old, already a legend — whereas the rest of the crew were still teenagers when they signed on. He's the gravity in the crew's orbit. The adult. The one who knows how badly things can go wrong, because he's seen them go exactly that way.

He was a member of the Sun Pirates under Fisher Tiger — the fish-man pirate crew that freed thousands of enslaved fish-men. When Tiger died, Jinbe was the one who held the crew together, who accepted the Warlord position to protect Fish-Man Island, and who stood at the center of the fish-man–human peace he'd spent his whole life trying to broker. He walked away from the Warlord title to help Luffy at Marineford. He spent years afterward under Big Mom's thumb because she refused to let him leave. And then Luffy showed up, and Jinbe did what he always does: he chose the right side.

He's fish-man karate incarnate — a martial art that weaponizes water itself, turning even air into liquid projectiles he can manipulate. He can walk across the ocean floor. He can steer the Sunny through any storm. And he can stand against Yonko and keep standing. Luffy didn't just add another crew member. He added a legend.