Poo (JP) is one of the four main playable characters of the game EarthBound. He hails from the foreign land of Dalaam, (Ramma in the Japanese version). He went under strict martial arts training, and knows a variety of PSI moves, eventually learning the powerful PK Starstorm. He was also put on a diet that restricts him from eating Western-style foods.
Poo first enters the story half way through the game. After eating a Magic Cake in Summers, Ness dreams of the faraway land of Dalaam. Poo is completing his Mu meditation training. The first piece of training involves blocking out the outside world. The player does this by not touching the controller and staying still, even when someone tells him there's a problem going on. Next he battles the spirit of ancestor. Poo has to let the ghost take his HP down to 0 to win the battle. Following this, Poo will complete his training. His master will teleport him to Summers and tell him to join Ness's party. Poo has a Tiny Ruby in his inventory that can be used to get into the Summers Museum.
Poo learns PK Starstorm, one of the most powerful PSI moves, usually considered tied with PSI Rockin, late in the game. He uses this to defeat Master Belch (By then known as "Master Barf") in the Deep Darkness swamp, at the time he'd temporarily left the group, coincidentally, to learn this move. Poo's special skill is the Mirror skill. This allows Poo to copy an enemy's attacks until the end of the battle, including attacks he previously didn't have. Another feature specific to Poo is his diet, which makes him restore less HP than others when eating Western food. To balance this, there's Eastern food that grants more HP to Poo as well.
From Super Smash Bros. Melee:
"Poo, the crown prince of the mysterious eastern country of Dalaam, joined Ness after completing rigorous training in his home country. While he doesn't stand out among Ness's friends, his vast experience and strong willpower make him a key to their success. Poo can assume the form of an enemy to gain its powers."