Amy M
A beautiful dark haired singer with a chrome arm. When in public, she tends to wear formal dresses

 Amelia Moxie [that's me] was a not very talented young torch singer, down on her luck. Master villain (though she didn't know that part) Maximilian Corduroy promised to give her an angel's voice, out of the goodness of his heart. Naive, she agreed and he conducted a procedure on her involving mermaid's vocal cords that succeeded in giving her a literally enchanting voice.

 It didn't take long for Amelia to gain quite a bit of fame singing around town with her new voice, and she got involved with the mob (also partially due to Corduroy's manipulations in the background). She spent several years with the mob, including helping Corduroy with several schemes and losing her naivete in the process. Eventually, man-of-mystery Dirk Derringer started working a case against the mob, and she saw a way out. She let him think he won her over with his charm and classic good looks to betray the boss's secrets to him.

 Corduroy expected as much, and captured her and used her as bait to lure Derringer into a trap (which had been his long game the whole time). Something involving a giant saw... Derringer of course came to her rescue in typical heroic fashion. However, Derringer was just a tiny bit too slow that day, and the saw cut deeply into her shoulder before he could rescue her.

 That's where you come in - you were near enough that you were able to stabilize her and built her the prosthetic arm. (I've been imagining you were working with Derringer behind the scenes, building gadgets for him in his "batcave", but staying out of the action...)

 As for Derringer, he disappeared during the rescue, blown out to sea and we both assume, killed. With no children or living relatives, you inherited his wealth and crime fighting apparatus, most of which you were responsible for building in the first place.

 Corduroy got away and is still at large.

 Out of guilt (on my part anyway), we decided to pick up where Derringer left off and do the crime fighting thing ourselves. In particular, we're trying to hunt down Corduroy and bring him to justice. We're doing the music thing as cover to move around the underworld a bit more unobtrusively to gather clues.