As their codenames suggest, Mr. Pi and Ms. Halloween are former members of Baroque Works who defected from the organization prior to the events involving the Straw Hat Pirates. Their current whereabouts and activities are unknown. They both possess devil fruit abilities, and as with each team in Baroque Works had their own theme; their theme was Horror.
They're both very cruel individuals who like to prolong the suffering of others, once their plans are set in motion they like to sit back and watch chaos unfold. Despite their strong personalities they seem to break down when suggested they are romantic with one another, and protest that their partnership is entirely professional.
Mr. Pi is tall, lanky and dressed as a mummy to follow the duos horror theme. Specifically he wears dress pants and shoes and an unbuttoned suit jacket revealing his bandaged torso. He wears several pieces of golden jewelry around his neck featuring hieroglyphics and the symbol for Pi. He walks around in a shambling manner.
Ms. Halloween is a petit young woman whose wardrobe is fitting for the holiday she is named after. She wears an orange and black dress with a pattern that looks like stitches. Her most striking feature is her orange hair, cut shoulder length with jagged bangs that resemble a jack-o-lantern.
While hiding out on an island where society was much like a western, Mr. Pi dressed like a cowboy and wore leather chaps, vest and boots, as well as a cowboy hat, and a sheriff's badge with the symbol pi on it. He thought himself "blending in" despite the fact he was still bandaged like a mummy. Ms. Halloween ridiculed him immensely for it. This was also during the time they fought the Windfall Pirates.
Mr. Pi has the power of the Plague-Plague Fruit, appropriate for his mummy motif, and gives him the power to control viral and bacterial diseases. He can also use his own body to engineer new diseases, changing what symptoms they produce and how effective they are. To spread these diseases he must either touch, cough or sneeze on his target. Mr. Pi can also unravel his bandages and ensnare targets, this power is unrelated to devil fruit but serves as an extension of himself in spreading diseases. As for drawbacks, it is unclear whether Mr. Pi is susceptible to his own diseases limiting him from creating super-bugs.
Ms. Halloween has the power of the Scary-Scary Fruit, giving her the power to shapeshift into her enemies greatest fears. The effects are quite potent when they work, and in theory give her temporary powers that could match zoan or logia fruits, should her target be afraid of snakes or darkness for example. But the fruit has several drawbacks: her fighting style is inconsistent as a shapeshifter, she may not know prior to changing what she will become, and her forms are limited to the fears of those around her and she gains those strengths and weaknesses.
The idea of a mummy dressed like a cowboy comes from the movie Bubba Ho-Tep.