I was going to write this in that "Sanji fans are Judge's sons and daughters" thread, but it was locked before I could finish. Thought I would share it here.
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Sanji topics are the most toxic topics on this forum, but I'll try to give my short, honest thoughts on him, and him alone.
Sanji is the chef of the Straw hat crew. He's a man who lives by his mantras to an almost absurd degree. These include his gentleman tendencies and principles on food, while also serving an important place as a combatant for his friends.
The first, and I would say most important trait of his is how cooking and feeding inform his kind and giving nature. When Luffy arrived on Baratie, he was looking for a cook, and stumbled upon Sanji feeding a starving Gin for free.
What convinced Luffy of choosing Sanji wasn't just his skills as a chef, but the indiscrimination behind serving someone. He wasn't just good at his craft, he was committed to it, and those are the qualities expected of a future Pirate King crewmate.
On the other hand, there is an irony to Sanji's character. Sanji's role as a chef is the most supportive role by nature, tied with Chopper's doctor role. It would be easy to mistake him as helpless in more distressful endeavors. Oda lampshades that using Fullbody, who believes Sanji's just a pitiful waiter and cook.
His expectations on Sanji not met, Fullbody gets his comeuppance from the Chef of Iron.
In these two scenes, Oda has defined who Sanji is at his core. As a cook, he's suppose to be the gentlest and most unassuming onboard, but in truth he's one of its most capable members.
And how Oda portrays that is through the utility he possess. Sanji can fight, he can spy, and he can protect. He is normally the guardian of the weaker members, bails out the crew from a jam due to quick thinking and resourcefulness, and can fight with the best of them.
Sanji doesn't have to prove much to me as a fighter because he's already going above and beyond what's expected of him, and he'll continue to be one of the stronger Straw Hats, and a man who can bring more than that to the table.