Jimbei and Luffy's relationship is so special to me. Like, Jimbei only knew him as Ace's little brother and he already established that that only went so far with him, but then Luffy freed him and they fought together and by the end of it Jimbei decided that he'd protect him with his life. He rescued him physically, but also mentally and emotionally. He saw Luffy bleeding his heart out and said, "I'll cradle your innards until you're ready to shove them back in through sheer force of will," and Luffy did. When they parted ways, they already planned to see each other again and, when it looked like they might miss each other, Jimbei sent a message so Luffy would still be able to find him. Literally shared blood, an act whose significance is exponentially increased by the fact that they're a human and a fish-man. Right in front of a kingdom watching them spit in the face of lifetimes of prejudice, Luffy asked Jimbei to join his crew, then went nuts when he asked him to wait until he wrapped up his business. He was immediately like, "I gotta live up to their expectations, earn my place in the crew, live by the straw hat, die by the straw hat". Huggies and piggyback rides. In the middle of all this, he apparently gushed so much about Luffy to his crew that they expected their own captain to leave them. When Luffy talked about the end of his dream, even though Jimbei thought it was ridiculous at first, he ended up taking it seriously because it was his captain's dream.
Jimbei's like, "I believe this man and his crew will achieve what the greater powers in the four seas have been attempting and failing to do for the last 40 years and more. He's the key to ending the oppression of our race and leading the world to the light. While he hasn't achieved his full potential yet, what he is now is enough for me to swear my undying loyalty to him. His cause is my cause. His name is Luffy-kun~."
