Mocha is much larger than an average little girl because of the experiments done to her. She has a large head that is not very proportionate to her body, big purple eyes, and wavy, unkempt, black hair. She wore a white colored sleeveless dress during her time in the Biscuits Room and a coat outside when Kin'emon gave them all warm clothes.
In a photograph from before she was kidnapped, Mocha can be seen at a normal size and wearing a dress with a flower pattern and a backpack.
She is normally a timid little girl, as she was scared of Kin'emon's severed head. She seems to like Nami, calling her "Big Sis". She was shown to be very determined, as even when the Straw Hats ignored her, she carried on pleading them over and over again to help her get out of the Biscuits Room and asked Nami to help the other children. Later she also likes to call Usopp "Big Brother" after he tried to protect the children. She has proven herself to be very brave after learning the truth about Caesar Clown's experiments as well as dedicated to her friends, evidenced by her swallowing the drug-laced candy to protect them, in spite of it critically injuring her internally.
Abilities and Powers
Having been scientifically modified by Caesar, Mocha's size and strength have been increased to match that of a giant's, though the continued experiments and drugging have left her body full of foreign chemicals that would eventually lead to her death if left unchecked. She was also strong-willed, being the first to snap out of the NHC10 addiction and to overdose on it and not die.
At some point, she was taken from her mother after being told by Caesar that she and the other children with her were sick and they need to stay to prevent their parents from getting sick. Caesar tearily told the children that he had a son, but he caught the disease the children have and died from it, which is why Caesar wanted to heal the children, earning the children's trust and sympathy. Mocha stayed with the other children in Biscuits Room on Punk Hazard, getting "treatment for the disease".
Punk Hazard Arc
Mocha was in the Biscuits Room when the Straw Hat Pirates found her and the other children. She and the other girls find Chopper an adorable animal. When she pleaded to Nami, Nami decided to help all the children escape. Mocha also told Nami and Chopper about the corpses in the walk-in freezer.
Later, she succumbed to the withdrawal symptoms of NHC10, which she and the other children were fed regularly in candy form. Usopp eventually knocked out the children with sleeping gas, and the Straw Hats chained them all up for safety reasons.
When Caesar Clown found Nami, Usopp and the children, Mocha woke up and was then seen breaking out of the chains and attacking Nami and Usopp due to the effects of NHC10. She then left with Caesar for the drug.
Back at the facility, she was left in the care of Caesar's henchmen, who were supposed to take Mocha and the other children back to the Biscuit Room. However, it appeared that Mocha was not completely overtaken by the withdrawal effect like the rest of the children and hesitated to follow the men. When spoken to, her withdrawal worsened as she hallucinated and saw a man's head turning into a snake and remembered Nami and Usopp's warnings.
Completely driven into a panicked state, Mocha refused to go with the men and stated that she wanted to leave, crying for Nami and Usopp to help her. Chopper later rescued her by getting rid of the guards and injected her with sedative drugs that neutralized her withdrawal symptoms temporary. Later, having developed some kind of resistance to NHC10, she helped Chopper try to keep the other children from going into the Biscuit Room by guarding the candy they were supposed to eat. During the ordeal, Chopper explained to Mocha about Caesar's true colors and the terrible experiments he has been doing to her and the other children. Mocha became devastated once she heard that Caesar had no intention of letting the children leave and the candy they have been taking will eventually lead to their deaths. When the children reached the Biscuit Room, Mocha tried to run away with the candy, but Monet blocked her path with a snow wall. When Mocha asked for her help, she replied that it is not nice to hog all the candy for herself and that she should share.
Afterwards, Mocha ran from the Biscuits Room with the candy, with the berserk children chasing her. As she ran, she thought of her time on Punk Hazard. Mocha then cried out, pleading to the children chasing after her to return to their normal selves.
However, in spite of the Straw Hat Pirates' best efforts, the berserk children managed to catch up to her. Remembering what Chopper told her about the "candies" being deadly over time, she swallowed the entire batch to prevent the other children from taking it. As a result, she suffered from a serious overdose and Chopper had to choose between sedating the other children or saving her.
Fortunately, Sanji and the Marines arrived, subduing and sedating the children for him. On top of that, they carried her to an examination room where Chopper could diagnose her and prepare an antidote. He promised to save her along with the other children.
Later, after successfully escaping from the lab, Law treated all of the children, including Mocha, although she remained unconscious for a while until everyone started having a celebration, when she is shown to have fully recovered, much to to the joy of everyone, especially Chopper.
After the party had ended, the G-5 Marines started to escort the children to the tanker to get them home. While they were doing that, they put up banners and started saying things like the Marines are justice and pirates are evil scum. Though the kids still thought of the pirates as their saviors, and wanted to say their goodbyes to them, the Marines responded by pointing their guns at them and saying that only bad kids wanted to see pirates. However, the kids still insisted that those pirates were the only ones who had ever come to this island and also rescued them without having a reason to do so. The Marines heard this, but still continued their propaganda, until Tashigi interrupted them and said they, the Marines, were a disgrace. The Marines then explained that if they did not badmouth the pirates, they would come to respect and admire them, which they should not do as Marines. Then the Marines finally lowered their banners, allowing the recovered children to give their thankfulness and goodbyes to the Straw Hat Pirates. The children also stated that they would also become pirates when they grew up, but were admonished by the Marines for doing so.
While Smoker was recovering from his wounds, Tashigi offered to bring him coffee and muffins. Mocha is with the other children as they excitedly requested food too. Tashigi also informed them that they will see Vegapunk in a few days.