Let’s talk about Shanks, Kid and how I think Oda intended for them to be similar, yet polar opposites ideologically.
(This is not a theory about how they’re related by blood. This is a theory discussing their character design, ideologies and how it plays into the themes of One Piece).
Starting with their appearance:
Things that are Similar: They both have red hair (duh), they both are missing their left arms, and have scars over their left eyes.
Things that are Different: Kid dresses much flashier, as opposed to Shanks who dresses very simplistic (this becomes important when we get to the ideology).
Kid: Arrogant, violent, savage, merciless, prideful and egotistical.
Shanks: Modest, tame, puts himself in danger for causes larger than himself, and doesn’t care for petty conflict.
How this all Ties Together (The Ideology Part):
Kid and Shanks represent how two people who fall into similar positions can react to them in different ways. When Shanks is bullied by some bandits at a bar, he laughs it off. When Kid is mocked for wanting to find the One Piece, he kills them. Or when they both lose their arms, Kid replaces his with a mechanical arm, but Shanks leaves his as it is. This has to do with how Shanks is simplistic and naturalistic. Shanks is all about the human spirit and will power. He doesn’t use technology or devil fruits to gain an advantage, he uses his natural abilities as a human (Haki). Where as Kid uses technology and a devil fruit. I feel Shanks would view these things as unnecessary or as excessive; he goes as he is. This bit of philosophy becomes clearer by their clothing. Shanks’s simplistic vibe is expressed through his casual/rugged attire; with his baggy pants, untucked, button-down, and sandels. Kid’s flashy pants, vest and coat are polarizing to Shanks’s wardrobe.
The way they treat their enemies is another way their character philosophies differ. Shanks and his crew could have taken out those bandits in the bar, right then and there. But he chose to show some mercy and not induce violence over something as petty as spilled drinks and a little name calling. But when Kid travels to the New World and defeats Haritsu Kendiyo and his crew, (which, presumably for Shanks would have been enough), but Kid shows no mercy and kills them.
Shanks and Kid also seem to face some similar hardships and situations, but the way they react/handle/cope is opposite to each other. Now there are a lot of characters in One Piece that are opposites of each other. But the reason I think Oda intended for a special and specific connection between them is their physical appearances and interactions with Luffy. We already went over their looks, but just to recap, they obviously bear similar wounds to each other. Now, Luffy takes after Shanks (the way he handles people, his philosophies, his values, ext.) and Kid is supposed to be Luffy’s opposite/rival. We know this because Kid was the only supernova to have a higher bounty than Luffy, he was the only supernova whose vice captain/first mate also held a bounty high enough to be classified as a supernova, and Kid’s giant metal arm was a parallel to Luffy’s gear 3 pistol. So Kid is an opposite/rival to Luffy. The reason this supports the relation between Kid and Shanks is because, for the sake of the story, it’s important that Luffy would meet someone who bares similar pain/experiences as Shanks, but handled it in the opposite manner; representing what Luffy could have become if he didn’t follow Shanks’s way of life (Kinda like Naruto and Gaara).
Oda intentionally wrote for Kid and Shanks to have some similar bad experiences to represent a theme about how having the right attitude can make all the difference in life. He purposefully modeled Kid to be noticeably (but not too obviously) similar to Shanks in appearance to create that parallel between the two characters.
*Theory by Fiasco