There's something to be said about how Eiichiro Oda, the creator of One Piece, is able to make his audience care about the biggest weirdos in his long-running pirate manga. There's always more behind the curtain to every character we meet in One Piece despite how odd or offputting they may come across, and no better character does this fact ring true with than the Straw Hats' resident shipwright, Franky. Oda was able to turn Franky from an unlikable act 1 villain to a complete oddball and, finally, one of the most sympathetic likable characters to come out of the Water 7 Saga.
Fans first met Franky through the Franky Family, a group of ship dismantlers and thugs who ganged up on Usopp and stole the 200,000,000 Beli he was carrying. Usopp, feeling he let down his crew, took it upon himself to go after the Franky Family alone and retrieve the stolen money. Franky not only defeated Usopp with ease but belittled the sniper, saying "I feel sorry for Straw Hat Luffy if the only one to guard his money was a good for nothing crewmate like you." Franky's cruel actions and masked face are a strong start to making fans hate him.
The next time Frankyis seen is when we get his full design. It comes as a shock to see that the person who hurt Usopp, is a freaky speedo-wearing cyborg with Popeye arms. Franky's off-color personality and goofy design also add a bit of levity to some otherwise heavy scenes. After just seeing the crew fall apart and Water 7 recovering from an assassination attempt, Franky's antics manage to bring back a lot of that classic One Piece charm. The moment where he's so upset with the Galley-La craftsman for stealing his fight against Luffy that he threw a coffee table in anger is a hilarious gag.
After Luffy and Nami escaped from Franky and the Galley-La Company, the cyborg still wanted his revenge. He planned to use Usopp as bait to lure the Straw Hats to him. However, things don't shake out the way Franky planned. The cyborg is moved to tears by Usopp's story of falling out with his crew. Then when Usopp was still in denial about the Merry, Franky knocked Usopp underwater for him to see the cracked keel for himself. Franky is the first person Usopp told about the spirit he saw on Skypiea, and Franky reassured him by telling him about the Klabautermann. Franky may be a bit "unorthodox" in his approach, but it's hard to completely write him off as a bad guy, at this point, especially once his backstory is revealed.
Franky was abandoned by his pirate parents as a kid, and taken in by the Water 7 shipwright, Tom. Alongside Tom and Iceberg, Franky helped to make the Sea Train meant to save Water 7 from ruin. However, a government agent from CP-5 named Spandam came to Water 7 to meet with Tom and obtain a set of blueprints that could be used to build an ancient superweapon. Spandam manipulated the people of Water 7 to get what he wanted and used Franky's own battleships to attack the city and get Tom re-arrested. When Franky revoked any attachment he had to his battleships, Tom punched him, stating that "a shipwright can never stop loving their creation whether they be used for good or bad". Tom is still taken prisoner to government island, Enies Lobby, but the audience saw how far Franky went to save Tom when he tried to take on the Sea Train on his own, doing irreversible damage to his body in the process.
The next important Franky moment came after he is captured by CP9 and placed in the front car of the Sea Train with Robin to be judged in Enies Lobby. Franky understood that if the government did get their hands on the ancient weapons and the blueprints, they could eradicate the Pirate Era and there would be nobody left to challenge them. However, Robin was okay with this as long as the Straw Hats would be safe. Robin only saw herself as a burden if she stayed on the crew, as she has convinced herself that she doesn't have the right to live. However, Franky objected to this, reminded of the words his mentor taught him, that "Existence is not a crime."
A running theme with Water 7 is the concept of "trust" and "deceit." Characters that were initially thought of one way or simply written off as nothing more, are hiding far more depth than anyone could have expected, and Franky is a perfect example of that. Franky's personality doesn't change between his first introduction and the current story arc. When he first got the money he gives 5 million of it to the Family to party with. At first, he simply wrote off the Straw Hats as a group of lousy pirates, but it's not until he listens to Usopp does he see they are more than that. He sees that these were people worth fighting the government for, which is exactly what he does by fighting CP9 alongside the Straw Hats and burning the legendary Pluton blueprints.
Franky is a goofy weirdo, but he's a person who's proud of who he is and empathetic to the people around him. Whether that be in how he took in the various members of the Franky Family when they were all outcasts, his friendship with Usopp, or the slow romance that's building between him and Robin. Franky's the SUPER shipwright of the Straw Hat Pirates and no one else is more suited to this merry band of pirates.