Lassie can be found next to Noah in Noah's Ark, standing or sleeping by his side. He uses her as a guard dog to keep him safe in his sleep as well as defending the Ark. She is not hostile unless the player attacks Noah or the dog itself. If Noah survives the game, then Lassie will give birth to several puppies and help defend the Ark from a Snork attack in one of the ending slides.
- Attacking Noah turns Lassie hostile to the player and vice versa.
- Its name may be a reference to a famous fictional dog which appeared in several movies and TV series.
- Noah completely forgets Degtyarev (even if he attacked him before) after he leaves the barge and moves to a reasonably large distance from it. This is not applied to his dog. It will remain hostile to the player for the whole game if he attacks any of the two.
- If Lassie is dead and Noah is alive the player will nonetheless get an ending slide stating that the dog gave birth to several pups and defended the barge from mutants along with them.
- The crosshair will turn red if the player looks at the dog, even though it is not hostile to him.
- It is one of the only two mutants to be tamed by a stalker. The other one is also a pseudodog belonging to Doctor.