|Out of universe information|
Richard Dean Anderson- (in Jack O'Neill's body).
Kanan is the Tok'ra symbiote who blended with Jack O'Neill temporarily in order to heal his body, after a virus attack from the Ancient Ayiana found in the Antarctic ice. He was on a vital undercover mission when his host was fatally wounded. (SG1: "Frozen")
After achieving symbiosis with O'Neill, Kanan was confronted with O'Neill's concept of never leaving anyone behind. Having fallen in love with Shallan, the lo'taur of Ba'al, Kanan made a desperate attempt to free her. However, the mission failed, and Kanan left O'Neill's body. After SG-1 managed to free O'Neill, thanks to some help from Daniel Jackson and Yu, O'Neill finished what Kanan started and freed Shallan, who joined the Tok'ra to fight in his name. The symbiote is presumed dead, but O'Neill's consequent torture further soured his already dim view of the Tok'ra. (SG1: "Abyss", "Allegiance")
It is unclear whether or not O'Neill retained a protein marker from his Blending with Kanan as it is never addressed in the series, it is stated by Carter that a symbiote must blend with the host in order to leave a protein marker (SG1: "Legacy"), it is also made apparent that the symbiote does not need to die in the host as Vala uses a Goa'uld healing device on Daniel after having her symbiote extracted. (SG1: "Prometheus Unbound")