Shamisen is a male calico cat. A former stray, he was picked up by Haruhi Suzumiya in order to play a role in The Adventures of Mikuru Asahina Episode 00. He now lives with Kyon. Kyon notes that Shamisen being a male calico cat is itself unusual, a 1 in 3,000 chance requiring the cat to have Klinefelter’s syndrome.
Fun Fact: A shamisen is a three-stringed musical instrument similar to a guitar, originating in Japan. A shamisen is usually made of cat or dog hide, so calling a cat “shamisen” is analogous to a Westerner calling a cat “tennis racket.”