One Piece: Fused with Puruni, I’m the Straw Hats’ Vice Captain

[Traveling with the crew] [Invincible]

Ji Shengling transmigrates to the world of One Piece. By a stroke of luck, he merges with Seer Puni's powers as he crosses over.

In the terms of the One Piece world... a Human-Human Fruit, Mythical Zoan, Model: Puni? Well, that could work. Except he doesn't have any of the drawbacks of a Devil Fruit.

From the start, he meets Luffy, a straw-hat kid who's just set out to sea alone. Ji Shengling immediately joins him and becomes the vice captain of the Straw Hat Pirates.

He helps Luffy break the sea-water curse of his Devil Fruit while strengthening the rest of the crew.

Sanji gets the Rumble-Rumble Fruit, Nami the Float-Float Fruit, Usopp the Door-Door Fruit, and Chopper two fruits.

He gives all the forces at sea a massive headache.

Facing [Nothingness], Akainu breaks into a cold sweat.

Akainu: Why can't even Haki-coated attacks hit him?

Facing [Elements], Blackbeard can't accept it.

Blackbeard: Impossible! Why can he make my Dark-Dark Fruit stop working?

Facing [Energy], even Shanks can't help but marvel.

Shanks: His Haki is unbelievably powerful. Luffy really found himself a great partner.

Facing [Life], even Kaido admits defeat.

Kaido: I acknowledge that you're the Strongest Creature!

As for the Celestial Dragons, the Five Elders, and Imu, Ji Shengling says: I'm the real god here. You useless fools, I'll beat you up every time I see you!

(Even if you've never played Seer, you can still enjoy this. The protagonist's weapon is a halberd inspired by Smoke Mirror from FGO, basically a pistol that can also be used as an axe. You don't need to know any of this to enjoy the story.)