One of the big things about The Tin Woodman of Oz is that we get to go back to the story of the Tin Woodman and finally meet some of the characters who were once just part of an anecdote. Baum would rarely go back to old characters who had been left behind. The Good Witch of the North, the Queen of the Field Mice, and even Jack Pumpkinhead vanished or quietly slipped into the background only to receive brief mentions or appearances in the later stories.
You may have noted that Baum took the liberty of revising the Tin Woodman's story in Tin Woodman. While it fits with his very good habit of not leaving audiences in the dark about his characters, this time, the information was crucial to the story. (And also to get the Tin Woodman's backstory otherwise would mean getting a copy of The Wizard of Oz, which was published by a different publisher and Baum had signed over the royalties to pay off debts.) This meant an entire character—Nimmie Aimee's mother—was culled from the proceedings and the Wicked Witch of the East given a larger presence in his tale.
Fyter also claimed one advantage over the Tin Woodman: he had brains and a heart. However, he noted that his brains were scraps of tin and when he thinks, they rattle, so he tries not to think. Also, his tin heart is cold and hard.
Captain Fyter joined the Tin Woodman on his quest and they soon found Ku-Klip's shop, where they met the tinsmith who fashioned their tin bodies. Ku-Klip is as kindly and helpful as ever, even offering to make tin legs for the Scarecrow.
However, as the inventor of tin prosthetics, Ku-Klip has a disturbing side. He viewed the creation of the tin bodies as exchange for the "meat" bodies and actually kept them. Remember when I discussed the Tin Woodman's human head in the cabinet? Yep.
The old Tinsmith was seemingly on good enough terms with the Wicked Witch of the East to have her glue a severed finger back on with "meat glue." This compound would join flesh and bones together perfectly, and after the Witch died, Ku-Klip took it. Deciding he needed a hand around the shop, he used the pieces of Nick Chopper and Captain Fyter to create a new man: Chopfyt. He had Captain Fyter's head, Nick Chopper's right arm, but no left arm because Ku-Klip could not find a left one for some reason.
Ku-Klip was also able to tell the Tin Woodman where Nimmie Aimee went, and sent him off in the direction of Mount Munch. But at Mount Munch, the group found a surprise. Nimmie had met Chopfyt. And married him. Her reasoning is that he reminded her of her lovers as they were before they became tin, and the tin arm reminded her of them afterward.
And none of these characters reappear in the Famous Forty Oz books. Melody Grandy's Forever in Oz liberates the Tin Woodman's head by giving it a tin body and creates a daughter for Chopfyt and Nimmie, though she notes that Forever is—due to biology—Nick Chopper's daughter. There's a very special ending for Nick, Captain Fyter (who Melody names Feersom), Nimmie, and Chopfyt, but I won't spoil that.