Chantho was a female Malmooth and the last of her kind.
Biography
Chantho left the Malmooth Conglomeration and worked with Professor Yana of the Utopia Project. She felt an unrequited love for Yana and confessed it to Martha Jones, who sympathised with her, having similar feelings for the Tenth Doctor. When Yana discovered that he was the Master in human form, he fatally wounded Chantho with a live electric cable. With her dying breath, she shot him, triggering his regeneration. (TV: Utopia)
When Davros asked the Tenth Doctor, "How many have died in your name?", the Doctor recalled Chantho's death. (TV: Journey's End)
Personality
To be polite, Chantho, like all Malmooth, had to say the start of her name at the beginning of each sentence, then finish with its end. When Martha asked what would happen if she didn't speak that way, her reply was, "Chan-That would be rude-tho", like swearing to humans. However, she was able to overcome this with effort, as she demonstrated to Martha, saying "...No", which both she and Martha found funny.
Chantho stated that she adored Professor Yana, possibly to the point of being in love with him, a trait she had in common with Martha who was in love with the Doctor. When Yana turned into the Master, Chantho tried to talk some sense into him but was finally forced to aim a gun at him. She couldn't bring herself to pull the trigger until Yana had fatally wounded her. As she was dying, she shot him and forced him to regenerate. (TV: Utopia)