Mr. 7 is a former Frontier Agent of the criminal organization Baroque Works, where he serves as half of a sniper duo alongside his partner, Miss Father's Day. He assumed the codename after the previous Mr. 7 died in the line of duty. His appearance is heavily themed around the number seven: his eyebrows, shoes, and gun are shaped like sevens, many of his hand gestures and mouth shapes also resemble the digit, and his clothing features repeated sevens. He has puffy bluish-white hair styled like a helmet, wears a purple coat with yellow frills and parallel sevens on the chest, square yellow sunglasses with red lenses, black trousers, long white socks, and yellow shoes in the shape of sevens. His typical laugh is "Ohahahaha."

Personality-wise, Mr. 7 often appears laid-back, frequently yawning or whispering to his partner, yet underneath he is ruthless and indifferent to the consequences of his orders. He seemed glad to have the chance to shoot Princess Vivi, whom he had previously known. His primary motivation is to follow the instructions of his leader, Sir Crocodile, without question. During the Arabasta Arc, Crocodile assigns Mr. 7 and Miss Father's Day to guard a cannon atop the clock tower in the capital city of Alubarna. Their task is to ensure the cannon fires at the appointed time to destroy the fighters in the square below, even though the blast would kill them as well. They attempt to snipe Vivi when she tries to stop the cannon, but the duo is ultimately defeated by her in combat.

Mr. 7 has no known bounty. His notable abilities include expert marksmanship with his custom Yellow Gun, a flintlock that fires dice-shaped bullets with seven dots on them. He coordinates precisely with Miss Father's Day so that their shots collide and create a large explosion. Beyond this teamwork-based sniping, he displays no supernatural powers or unique combat techniques. He does not undergo any significant character development and disappears from the story after his defeat.