Let’s start with this panel which is from over 25 years ago and contains the 9 Red Scabbards mostly in their youth or younger years.
One of them, specifically Kiku does not seem to appear here. It looks as if the one next to Denjiro is none other than the 16th Commander of the Whitebeard Pirates Izo.
So what backing does this theory have?
1- Izo is the 16th Commander, making me think he is one of the more “recent” additions to the Whitebeard Pirates
This isn’t exactly backing, but to think the last Commander he added was 25ish years ago seems alright since he was old and already a Pirate/Yonko. The people in the Fleet (like Doma who was forced by Ace) might’ve been chosen to stay as fleet members due to being “full” on Commanders. Supported by the notion that Ace took up the last of 2 remaining Commander slots, the other being the cook, Thatch, who they might have wanted to “retire” his Commander number, like in sports, due to his death (Zehahahaha).
2- Sometime over 20 years ago Oden, Inuarashi and Nekomamushi left Wano illegally to set sail with Whitebeard
It’s not difficult to think Izo left with them leaving his slot in the Red Scabbards open, as he was not seen at anytime in the flashbacks leading up to the time jump, since Kiku was already in place as a Red Scabbard, in my mind replacing Izo. This fits the timeline at least.
This would give reason for Whitebeard to trust Izo, maybe Oden vouched for him since we found out Oden “wooed” Whitebeard and Roger. So he decided to stay and become Whitebeard’s son and became promoted to Commander thanks to his relationship with Oden. He could’ve started out as a cabin boy like Shanks. (Possibly because he didn’t have a father?)
3- As we know now Izo originated from Wano, thanks to an SBS question and the Vivre Card Databook
Oda made a point to specifically pick Marco, Jozu, Vista, and Izo to draw as children. There’s no reason in my mind I could think of why Oda would draw the possibly weakest, possibly irrelevant 16th Commander Izo than his possible upcoming story relevance.
4- How does this all tie together?
Marco was sought after for aid in Wano via Nekomamushi. I would’ve blown it off if Marco had denied coming offscreen (and if Izo wasn’t involved now in this theory). But since Oda decided to show Marco and give reason to him not coming to Wano, it makes me think there’s no way he’s not coming to Wano if the reason is good enough.
So Marco’s “brother” Izo will call out to Marco for aid in Wano, thus giving Marco enough reason to leave the island and come to Wano. To help his “family” in his homeland, his past crewmates/friends Inuarashi and Nekomamushi, and aid Luffy in his journey to the top.
*Theory by -RedditCat-
