Moo-Chee-Goo-Chee-La-Poo-Chee the Third, more commonly known as The Dark One, was one of the first students of the Beifong Metalbending Academy. He had a severe distaste for everything, even going as far as announcing his hatred for everyone and anything. The pinnacle of his hatred was his own real name, and thus he made sure that it was never spoken.
Having deemed the name his parents gave him to be ridiculous, The Dark One soon developed an aversity toward everything in the world. One time, when he was out on the streets of Yu Dao uttering hateful remarks at the address of every passerby, he inadvertently moved Toph's space bracelet ever so slightly. The earthbending master thus recruited him for her academy, thinking he had potential to become a metalbender. However, progress was slow, as he was unable to move even a single coin by the time that Sokka visited the academy.
When the academy was taken over by firebender students of the Master Kunyo's Firebending Dojo, The Dark One commented that although he hated practicing forms like Toph had ordered, he hated being kicked out of their own school even more. Having managed to buy the students three extra days to practice, Toph and Sokka tried many different methods to get The Dark One and the two other students to bend metal. During one of these attempts, Sokka figured that since the students had originally moved Toph's bracelet unintentionally while in extremely emotional states, all they needed to do was recreate that state. To achieve this, Toph uttered The Dark One's real name, Moo-Chee-Goo-Chee-La-Poo-Chee the Third. However, instead of being able to earthbend, The Dark One broke down, stating that the mention of his real name had "lodged a splinter in [his] soul".
The evening prior to the battle, he and the other students were prepared to leave the academy when they overheard Toph telling Sokka that she had been expecting them to be something they were not. Inspired by the fact that someone expected him to be something more than "a tall, dark, mysterious hunk," he and the other students figured out the key to metalbending and were able to bend metal coins, thus defeating the rival firebending school.
He stated that he had the "soul of a poet", and thus started speaking in vivid and poetic terms, much to the annoyance of Toph, Penga, and Ho Tun, who all wished he would go back to hating people. When Toph was in trouble during the battle for Yu Dao, The Dark One, Ho Tun, and Penga helped out and metalbent the Fire Nationsoldiers' helmets.
After Toph departed from the academy to see her friends, The Dark One, Penga, and Ho Tun became its temporary instructors. He told the students that chi clung to their bodies like a heavy, suffocating cloak, and that they should take hold of its "dark folds" and keep guard against "the impish breath of distraction", confusing them. At that moment, Sokka arrived, telling the three that they needed to come with him. The Dark One told the students to continue their training and told Sokka that he was filled with questions, to which the warrior responded by saying he would explain more on the way, but it was a situation in which Toph needed their help.
The students arrived to the town where the Earthen Fire Refinery was located, where they met up with Aang, who explained the situation to them: Toph and several others were trapped underground and he could not dig them out due to the iron ore deposits in the ground; he thus needed their metalbending to do so. Katara added that Toph stated it to be like spider ants stomping beneath their feet, much to The Dark One's dismay, because it was an exercise in which they were blindfolded and had to stand in a sand pit while throwing giant metal plates back and forth over their heads, an exercise which, he added, if they failed to complete, they would be stomped by the plates. As Sokka noted that the response was a departure from his poetic ways, The Dark One explained that he could not compose poetry when freaked out of his mind. When asked by Ho Tun if he was ready, the poet said he could feel the iron beneath them and began bending on Ho Tun's call. Upon revealing a giant iron mask, The Dark One wanted to know what they had uncovered, to which Aang said it looked like the mask of a warrior or spirit. The metalbenders continued removing the iron and, with one final heave along with Aang, pulled everything out of the ground, allowing the caved-in group their freedom. Toph complemented the three, but after she collapsed into her father's arms, The Dark One called the situation "dread upon dread" and that "dark fog of despair [rolled] relentlessly through [his] heart". Lao assured them that she was just exhausted and would be fine. He asked who the man was, to which he confirmed his identity as Toph's father.
The students continued to stand near their instructor as she rested. However, when they learned that Aang was about to destroy the Earthen Fire Refinery and Toph awoke, she called on them to help her stop him. They raised an earthen wall to stop his attacks. They were blown off the wall by Aang's air rings but landed softly on their feet before returning to the battle. Toph called on them to metalbend, but their attacks were dodged. The Dark One soon looked strangely when the giant spirit, General Old Iron was approaching them. With an intense battle between the spirit and Avatar, Toph again called on the three, but this time to help Aang by peeling off Old Iron's armor with their bending, just like the forest bowing to the east wind exercise. They were able to do so and thus give Aang an open shot to the spirit's chest, but Old Iron soon noticed that the four metalbenders were destroying his armor, thus turning his attention them. Toph told them to run while she held him off, but The Dark One did not want to, saying the bitter grip of death was sill sweeter than disloyalty. They soon watched as Aang delivered a severe blow to Old Iron and the vanquished spirit gave a speech about how humans were more powerful than him before walking away. The next day, the poetic metalbender sat with Yee-Li while eating food, sharing a conversation.
Three months later, The Dark One participated in the parade for the new Spirits' Friendship Festival, using his metalbending to play the cymbals, floating them in the air above him as the procession made its way down the town's main street.
The Dark One was quiet and rather spiteful. He claimed to hate everything, especially his real name and his youth, and was resentful of people seeing him as just a pretty face, claiming he had the soul of a poet as well; he proved this point by reciting poems even in the midst of battle.
The Dark One was an earthbender, but from Toph's perspective, he needed practice in order to begin learning metalbending. Along with his fellow students, The Dark One was later able to teach himself a basic level of metalbending being able to redirect metal coins in the direction of Kunyo's students. During Toph's absence in the Beifong Metalbending Academy, The Dark One functioned as a temporary instructor, tutoring newer students in metalbending. Alongside Toph, Ho Tun, and Penga, he was able create a weak spot on General Old Iron by metalbending his armor off.