Toshiko "Tosh" Sato, portrayed by Noako Mori, first appeared in 2005 in the Doctor Who episode "Aliens of London". She is reintroduced as the technical expert at the Cardiff branch of Torchwood. 

She has a long-standing crush on Owen and they eventually go on a date. Unfortunately, Owen dies, is brought back to life, and then dies again before they can pursue their relationship. 

Tosh also has a relationship with Mary, an alien that unfortunately is using her to gain Torchwood weapons technology. She also gives Tosh a pendant that grants her telepathy. 

Tosh dies in the second season finale within minutes of Owen dying.