Moka accidentally transmigrated to the world of pirates and was adopted by Makino, the tavern proprietress. From then on, the trio became a quartet.
“Now that I've transmigrated, should I keep a low profile?”
“Ding! System binding successful.”
“Luffy is destined to be Pirate King! I say so, and no one can change that!”
If you're going to make it in this world, you need strength.
“I've eaten multiple Devil Fruits, I can use Haki and swordsmanship, and I can also...”
And you need connections.
“I'm the vice captain of the Straw Hat Pirates. I can call Garp the Hero of the Marines ‘Grandpa,’ and the Revolutionary Army's second-in-command and Whitebeard's Second Division Commander ‘Big Brother’!”
Moka: “Look after the ship, or I'll break your legs.”
Looking forward to further development