It's going to be Jinbe and Kinemon.
Jinbe I'm certain of, Kinemon I'm… strongly positive about.
I'll argue about Jinbe if anyone wants, but for now I'll focus on Kinemon. I don't think he's Momonosuke's actual father. I think his real father is the emperor or shogun of Wano. I think Kinemon, Kanjuro and the other samurai are Momonosuke's bodyguards who are shepherding him away from Wano because there is either a coup going on, or a pirate invasion is taking place and has made it unsafe for Momonosuke to be there. There's enough weird scenes that imply Momonosuke is hiding something rather important about himself, and a few moments where he is away from Kinemon that he addresses him oddly. If nothing else, the two of them have a secret that they haven't exposed to the Strawhats despite everything Luffy's crew has done for them.
Remember in the flashback that showed Kanjuro being captured, Kanjuro emphasized that Kinemon focus on Momonosuke, to the point of self-sacrifice. Momonosuke's aversion to food until he sees Kinemon eat might be that he's had to be wary of poisoned food due to a life at court. Well, I'm stretching by this point. I'll move onto why I think Kinemon would work.
Kinemon fits into the Strawhats rather well. Although he'd be the third swordsman, consider: Zoro uses wind attacks, Brooke uses ice attacks, Kinemon uses fire attacks. Zoro uses a self created three sword style, Brooke uses a western influenced one sword style, Kinemon uses an eastern influenced two sword style. In a group shot, he looks good in contrast to Brooke. They are nearly the same height, but Kinemon is more colorful and expressive. He helps balance out the asymmetry that the Strawhats have, much like how Jinbe's wide frame balances with Franky's. He is a pervert like Sanji and Brooke, but it's a different kind. Brooke is a dirty old man, Sanji is a love sick white knight, Kinemon is a lecherous man pretending at chivalry.
Of course, there's also the Devil Fruit numbers theory, which Kinemon fits, assuming his power is the Fuku Fuku Fruit.