Kosey was a gladiator who had trained under a Greek lanista Thanasis since the age of eight, eventually becoming his best fighter. At some point before 52 BCE, Kosey met and fell in love with an Egyptian woman named Hesper. Together, they had a daughter named Zalika and settled down in the Green Mountains region in Kyrenaika, near the lake in the mountains. However, due to Kosey's job as a gladiator, he was often away from his family, leaving Hesper to raise their daughter alone.
Around 47 BCE, Hesper became pregnant with a son. Kosey, desiring to spend his time with his family and watch his son being born, negotiated his freedom with Thanasis, who refused due to Kosey's skills and fame. As a result, Kosey decided to collaborate with the bandits in the mountains to ensure his release. During the journey to Cyrene's gladiator arena, Kosey faked his and Wamukota's own abduction, with the bandits capturing Wamukota and giving a horse to Kosey, allowing him to return to his family.
Meanwhile, their trainer Egidio fled and returned to Thanasis in Balagrae, resulting in the lanista sending the Medjay Bayek of Siwa to investigate. Bayek, having rescued Wamukota and receiving information of Kosey's whereabouts, travelled to the lake to find him. Kosey, believing Bayek to be sent to bring him back, attacked the Medjay, but was stopped by his wife. Bayek, learning of Kosey's reasons for leaving Thanasis' service, travelled to the arena in Cyrene to negotiate a deal with Thanasis. However, Thanasis went back on his word and upon learning of Kosey's whereabouts from a returned Wamukota, hired several Roman soldiers to bring Kosey back.
Kosey refused to return to Thanasis' service, resulting in him being captured. Before Thanasis could torture the gladiator, Bayek arrived, helping Kosey to defeat the soldiers and kill Thanasis, thus securing Kosey's freedom. Kosey and Hesper expressed their gratitude to Bayek, declaring to name their son after him. Kosey, having given up his job as a gladiator, also gave Bayek a shield.
- Kosey is an Egyptian name meaning 'lion'.