Fifi is the rather cute name given to an otherwise savage creature belonging to Helen A. of the Terra Alpha colony.
Terra Alpha is an Earth colony, so this creature is presumably a native of the planet that has been colonised. Helen A. kept Fifi locked in a small cage, but she was able to take it out and stroke it like any other pet animal.
Fifi was sent, on several occasions, by Helen A. into the pipes underneath the Terra Alpha colony to chase the pipe-people, and also to chase after Ace.
Fifi did manage to survive an explosion in the pipes caused by Ace's nitro-9, and was nursed back to health. However when Fifi went after the Doctor he led it to an area that held a lot of crystallised sugar. Fifi's howl resonated at just the right pitch causing tons of sugar to come crashing down. Fifi did not survive this. Despite Helen A's insistence that everyone on Terra Alpha should be happy the death of Fifi was too much for her.
- Terra Alpha