Hirudo Crescent is a half-Saiyan, half-human descendant of Sephiroth who was trained by Goku to become strong enough to fight while searching for his brother. He is the younger brother of Nero Crescent.
Hirudo was born in an unknown country south of Japan, the youngest son of Joshiro Crescent and Mikazuki Tsuki. His father had been a son of one of King Vegeta's scouting troops, which passed the Saiyan blood to himself and Nero. Joshiro was killed by Frieza during a spy mission prior to the discovery of the Dragon Balls, a few weeks after he was born. Hirudo and Nero, raised by their mother, were always fun-loving, hyperactive children. When he turned three his mother told his brother the story of how their ancestor, Sephiroth, had been killed in the crossfire of Madara Uchiha and Hashirama Senju's war in Konoha many years ago. He was surprised at this revelation, but didn't understand the full concept of it, which Nero had scorned him for. After Nero had sworn vengeance on the Konoha clans, Senju and Uchiha, Hirudo grew further apart from his brother and closer to his mother. One night, he caught his brother in the family's hall of swords, stealing Sephiroth's sword. Before Hirudo could call his mother, Nero ran up to him and used a sleeper hold, saying "I'll be back, little brother. I need to find someone to teach me how to become strong enough to avenge Sephiroth. Take care of Mom for me, you're the big kid now.", and Hirudo fainted as Nero set off into the night. After hearing of Nero's departure, Mikazuki took Hirudo and Cloud's sword, and set off for two years in search of her son. (Cloud's sword was won in the final battle of Cloud and Sephiroth and became tradition to be passed to the youngest Crescent sons, eventually falling into Mikazuki's ownership as Joshiro was an only child.) Hirudo was now five years old and having searched for two years, his mother contracted a terminal illness that killed her in a matter of seconds. Her dying wish was for Hirudo to take Cloud's sword as a gift and find someone to teach him how to fight and never give up until he found his older brother. Hirudo accepted this request as his mother died in his arms, ceasing his childish attitude and became more akin to Nero's feelings toward Konoha. He held a funeral for Mikazuki and set off alone, Cloud's sword in tow, in search of a teacher and his brother. He eventually met Gohan, another Saiyan/human hybrid that was in his teens. Gohan took Hirudo to his father Goku, one of the last surviving, full-blooded Saiyans. Goku took in Hirudo and immediately accepted the request to be the boy's teacher. He trained Hirudo for years, teaching him the Kamehameha, Spirit Bomb, Kaioken, and other useful moves. His mother's gift, Cloud's sword, became an essential part of his arsenal, having learned to channel his Ki into to make several miniature Destructo Disks, in a manner similar to Jon and Ichigo's Getsuga Tensho. Hirudo finally completed his training at the age of fourteen, and set off in search of his brother. He came across a rogue ninja named Sasori, who cast a Jutsu on Hirudo before he vanished. Hirudo didn't feel as if the Jutsu did anything, but over time became more ruthless and impatient in his attitude, brushing aside any concern for his ancestor's death and began feeling more animosity toward his brother. He reunited with Nero two months later, after the latter had taken over a country and renamed it "New Jenova", and told him about their mother. Hirudo was made Nero's second in command and general of New Jenova's military, but he had met the leader of Akatsuki and secretly helped find new recruits to build the New Akatsuki, while tricking Nero into believing he was the innocent, good-willed brother he had once been. (This evil attitude was a side effect of Sasori's Jutsu.)