his best friend
Well, that's one of the correct answers.
Hobbes routinely pounces on Calvin when he comes home,
hard enough to knock his socks off,
apparently doing significant damage, or at least getting his clothes dirty.
See
It's a Magical World
for a slow-motion replay of an attack.
The collection
Homicidal Psycho Jungle Cat
is named after a Sunday strip
in which Hobbes attacks Calvin on his way back to bed from getting a drink of water.
Artwork © Watterson/UPS; click for notice.