Tomoo was a young boy about 10-11 years of age and the ringleader of the group of bullies in the orphanage, which was also home to Lucy.
He is a key figure in her descent into madness, murder, and misanthropy. Due to the bullying he and his group inflicted upon her, Lucy developed a deep hatred of the human race, and he and his friends were the first victims of her vectors.
He and his friends were merciless in taunting, teasing and occasionally assaulting the friendless Kaede/Lucy, whose control over her emotions only drove them to ever-crueler depths, mocking and marking her with the nicknames 'Horns' and 'Ox' and likening her to an Oni. One day, after one of their assaults, another young girl befriended Lucy. The series makes it unclear whether or not this girl was sincere or did this at Tomoo's request to trick Lucy. But whether she willingly gave up the information, was fooled or just could not keep her mouth shut, the girl revealed to Tomoo and his friends that Lucy had been caring for a stray puppy. Tomoo found the puppy, brought it before Lucy, and slowly beat it to death while his friends held her. This pain and deception were all too much for Lucy to handle, and in a fit of rage, she unleashed her previously-dormant vectors and tore all four children apart, much to Tomoo's horror since she saved him for last. Later, when Lucy fears betrayal by Kouta for Yuka, she hallucinates that a headless Tomoo is still mocking her, telling her the world is meant for humans and not her. This singular act of pointless cruelty would in many respects define the rest of Lucy's life.
Lucy herself seems to understand the miserable need of Tomoo and his bunch to lash out at anyone more miserable than themselves. His cruelty stands in sharp contrast to the unexpected kindness of Kouta and others. It is unknown what Tomoo may have turned out like in the future, but it is worth noting that his gleeful torturing and animals and vicious bullying of the weak are classic serial killer traits. It is ironic that the victim Lucy and not Tomoo ultimately became the infamous serial killer.
Tomoo is erroneously referred to as Tomohiro in the original English credits from ADV.
His name can also be spelled Tomou or Tomo, and the kanji of his name can individually be read to mean "Satoshi" and "male," respectively.
Tomoo's character has similarities to Kakihara from Ichi the Killer and Chaka from Black Lagoon, as well as other psychopathic characters. Tomoo's most memorable act is the brutal murder of a lonely child's puppy. This act is not is not unique to Tomoo. He shares this misdeed with the classic literary sociopath, Pavel Smerdyakov.
Could Tomoo have been a better person in a different circumstance?