Kio Sr'thanu is a former soldier of Morturii who turned Kasiri after fleeing from King Helain's justice. He is 27 years old. Before Liall puts the Longspur krait under Peysho's command, Peysho reveals to Liall that he met Kio while they were serving in the Morturii army. Kio was sixteen at the time. They became easy friends, but no more than that, until Kio was sentenced to a brutal flogging that probably would have resulted in his death, if Peysho had not intervened. Kio had been caught giving oral sex to another soldier in the Ankar barracks, making him a sekech, a particularly despised type of man in Morturii. Peysho rescued Kio before the sentence was carried out.

Despite the difference in their ages, Peysho and Kio became lovers and kept on the move for the next six years, roaming throughout Byzantur, Chrj, Volstland, Minh, the Bledlands, and Khet in many outlaw camps and even on the seas. They settled with the Kasiri in Omara and moved with Liall into Kurstan, then the Traderlands of Aten and Elvyevah, finally moving into western Byzantur. By the time Scarlet and Liall meet, Peysho and Kio have been with the Kasiri under Liall's command for about five years.

Kio is recognized as an artist with a blade of any type. This wins him a certain respect among the Kasiri, and he is charged with weapons training in the krait. The Kasiri commonly refer to him as a "knifeman". In the chain of command, he is the enforcer's second, and thus third-in-command of the krait.

Kio is described as "deceptively kind of face and slight of body" with wide, tawny-gold eyes like a lion and feathery chestnut hair. He wears a light scattering of beard to make himself appear more masculine, but Liall notes that Kio's face is "as sweet as a girl's".

Few know the exact details of Kio's family connections, but Kio's speech, his manners, and his obvious education mark him as someone who was raised in a noble household. Liall admits to himself that he feels some attraction for Kio and envies Peysho's happiness, but he also realizes his feelings are futile.

It is revealed in Scarlet and the White Wolf: Book 4 that Kio has an older brother, Navan, who considers Kio his enemy for destroying their family's name.