This is Mortimer, named when the vet’s assistant said male, not renamed when another said female.

Mortimer is a small, shy cat who only comes out to play when everything is quiet.

A rubber band and an office chair provided a fifteen minutes of amusement.

@jean Apparently there's a book with a cat named Mortimer, but I've never found it.

@cliffordbeshers In case you're still looking, this is what Bard had to say:

• A Cat Called Mortimer by Shane Conaty is a children's book about a rich cat who is unhappy with his life. He decides to run away and find a new home, where he can be himself.
• The Cat Who series by Lilian Jackson Braun features a Siamese cat named Mortimer who helps his owner, Jim Qwilleran, solve mysteries.
• Mortimer by Robert Cormier is a young adult novel about a boy who befriends a stray cat named Mortimer. Mortimer helps the boy to cope with the death of his father.
• Mortimer's Tale by Charles De Lint is a fantasy novel about a cat named Mortimer who is the last of his kind. Mortimer must find a way to save his people from extinction.

@jarrod It turns out to be none of the above! I asked my brother and he said it was Mortimer, by Robert Munsch, so not a fictional cat after all, but a boy who won't go to sleep.