Kiyomi Takada knew his name from Light Yagami, who overheard his father say it in the raid on Mello's hideout. Before Soichiro died in the explosion after being shot by one of Mello's henchmen, he used his Shinigami Eyes to see Mello's real name. Light would have killed Mello himself with this information, but he had only the name and did not see/know Mello's face in order to kill him with the Death Note.