Chemo is first seen in the campaign mission "The Hornet's Nest", with Captain MacTavish, Ghost, Roach, and Rocket running through the favela to the evacuation site. He is later seen in "The Gulag" and will randomly appear throughout the campaign. Depending on the player's actions, Chemo can eventually be killed. He can be seen as a Canadian Task Force Operative, or without any nationality emblem. (Possibly American).
Chemo and Rocket will both always be replaced by another Task Force 141 soldier (if they die) up until the player reaches the market.
Even if Chemo dies in "The Hornets Nest", he still might show up in other levels as a randomly-generated character.
Chemo will always spawn with an SMG.
There is a Chemo in Call of Duty: Modern Warfare 3 and spawns as a Delta Force operative in Survival Mode. This could mean that Rocket was the one killed, and therefore Chemo survived throughout the game and was redirected back to Delta Force when TF-141 was disavowed.
Despite the player's actions, either he or Rocket will die in "The Hornet's Nest".