Cath is Smithy's mum. She is seemingly always depressed and doesn't care very much about her son. Cath has a mild form of narcolepsy and often falls asleep, however many people (including her daughter) attribute her inability to stay awake to her heavy drinking.
It is said by Pam that Catherine force fed her children 'Findus Crispy Pancakes', ate a lot herself, then threw them up, which is her explanation of why Smithy is overweight and his sister is thin. It is also said that she had an affair with a woman called Pauline, who was the headmistress of the school she worked at and was paid to keep quiet about the relationship. It is implied in one episode that she still sees Pauline; whether as a lover or just a friend is not specified.