It’s mostly wishful thinking, or just adoring Ulti for her design and personality. There is a low track record of villains joining Strawhats.
Robin would be a good example. She saved Luffy for her own reasons after he lost his first fight with Crocodile. You could say it was sympathy or her seeing Luffy as the key to Croco’s downfall. Now you could say that she did get rough with Pell and Igaram, but she was honestly going easy on them and staying “loyal” to Croc to avoid being caught. Now her joining the ship was the nail in the coffin since she felt thankful for Luffy saving her from the collapsing tomb.
Franky is another complicated example. He stole money from the Strawhats just to take a trip, let his Franky Family party, and give it to his favorite bar. Fought with Luffy publically to defend his “finders keepers” policy despite also causing a ruckus in front of Galley-La (collateral damage galore). It wasn’t until he got defeated and arrested by Cipher Pol that the Strawhats held a truce with the Franky Family to get him back from Enies Lobby. And it was after Franky escaped with Coup De Boo that Luffy would entrust him to keep Robin safe before him and his crew would make it to them. Franky decided to repay the favor by building the Sunny as a final thank you gift and end his shipright career, until Robin along with Luffy would play the “pants down” gig to try getting him to join the crew and not let his dream die.
Robin would be a good example. She saved Luffy for her own reasons after he lost his first fight with Crocodile. You could say it was sympathy or her seeing Luffy as the key to Croco’s downfall. Now you could say that she did get rough with Pell and Igaram, but she was honestly going easy on them and staying “loyal” to Croc to avoid being caught. Now her joining the ship was the nail in the coffin since she felt thankful for Luffy saving her from the collapsing tomb.
Franky is another complicated example. He stole money from the Strawhats just to take a trip, let his Franky Family party, and give it to his favorite bar. Fought with Luffy publically to defend his “finders keepers” policy despite also causing a ruckus in front of Galley-La (collateral damage galore). It wasn’t until he got defeated and arrested by Cipher Pol that the Strawhats held a truce with the Franky Family to get him back from Enies Lobby. And it was after Franky escaped with Coup De Boo that Luffy would entrust him to keep Robin safe before him and his crew would make it to them. Franky decided to repay the favor by building the Sunny as a final thank you gift and end his shipright career, until Robin along with Luffy would play the “pants down” gig to try getting him to join the crew and not let his dream die.