Uni

No physical description or design has been given for him anywhere in the manga or its supplemental material.

Nothing specific is recorded, but being part of the Heart Pirates implies real loyalty to Trafalgar Law and the same disciplined, professional mindset the crew relies on for its specialized underwater operations.

Nothing is known about his family or childhood. His origin is likely North Blue, where the crew was founded. He joined the Heart Pirates at some unspecified point and stayed with them through the two-year timeskip while Law operated as a Warlord of the Sea.

Wano Country Arc

Uni came to Wano as part of the Ninja-Pirate-Mink-Samurai Alliance, helping infiltrate the country and fighting in the Raid on Onigashima that ended with Kaido and Big Mom's defeat.

Winner Island Ambush

After leaving Wano, he and the crew were ambushed by Marshall D. Teach and the Blackbeard Pirates at Winner Island. Uni helped defend the crew's submarine, the Polar Tang, working with Shachi, Penguin, and Hakugan to fend off boarders and navigate the chaos of Blackbeard's Tremor-Tremor Fruit. The Polar Tang was ultimately destroyed and sunk; Bepo managed to pull an incapacitated Law to safety, but Uni and the rest of the crew were left in the water after the defeat. His current whereabouts aren't known.

Being part of a high-bounty New World crew, Uni is presumably a trained combatant, though his specific physical abilities aren't recorded. Like the rest of the Heart Pirates, he's skilled in maintaining and operating the Polar Tang, the world's most advanced pirate submarine.

Heart Pirates vs. Animal Kingdom Pirates — Raid on Onigashima

The alliance won, ending the reign of two Emperors.

Heart Pirates vs. Blackbeard Pirates — Ch. 1063–1064

An all-out battle at Winner Island sank the Polar Tang and defeated the Heart Pirates, effectively ending their current New World campaign.

No chapter is indexed as a major appearance for him by name. His presence is implied as part of the Heart Pirates' collective actions across Chapters 810-816 (Zou), 903-1057 (Wano), and 1063-1064 (Winner Island).

Trafalgar Law

Trafalgar Law is his captain, whose tactical orders he follows even in life-threatening situations.

Bepo, Penguin, and Shachi

Bepo, Penguin, and Shachi are his senior crewmates.

Heart Pirates

The Heart Pirates are his crew and family at sea.

No dialogue is recorded for this character.

His name, Uni (ウニ), is Japanese for "sea urchin," fitting the Heart Pirates' habit of naming members after sea creatures or polar animals - Penguin, Shachi (orca), and Bepo (bear) among them. He was one of many crew members who considered the Polar Tang home before the Blackbeard Pirates destroyed it.