John Irons: “What is Luffy’s mother like? Was she beautiful? What happened to her? Did she die when Luffy was a baby?”
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Oda: “I think she’s alive. I’m still thinking hard about this. [Laughs] But if she does appear in the story, then she’ll be a very tough-looking woman. And strict. There’s no way that she’s a beautiful mother. She’s got this typical middle-aged woman’s permed hair. Speaking about families, I did a backstory for Nico Robin and her family.
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That was actually the first time for me to draw a mother in a character’s memory. Up until that point, I’d never drawn a blood relation. In Chopper’s case, he’s called a son even though he’s not blood-related. So I’ve put in a message there saying that you can still call someone who’s not related by blood your family.” (American Shonen Jump, December 2009)
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Looking at this interview, we can see how the description of Luffy’s mother fits perfectly with Dadan 😀
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Dadan was introduced in April 2010, and maybe Oda decided to use the description for Luffy’s mother someone who played Luffy’s mother.
I mean Dadan is a very tough looking and strict person, She is definetly not a beautiful mother and she has got this typical middle-aged womans permed hair.