Sherlock Holmes likes to turn into a furry black cat by using a transformation potion. His best and probably only friend John Hamish Watson believes that he can also take an animal form without the magical drink but he tells John that he doesn't see a point in becoming an Animagus if he cannot speak while transformed. Cat Sherlock is often seen perched on John's shoulder but nobody else knows its true identity; people think he's only purring when he talks in his deep voice. It comes handy so Sherlock has no complaint but apparently John has. He says people will think him awfully lonely if he is seen discussing school works with a pet. Sherlock dismisses such a petty protest so in the end John speaks and listens to him indifferently no matter what shape he is taking.
Since nobody notices him as a cat, Sherlock sometimes visits John in his dormitory at night. He waits outside of the portrait hole for someone to open it then slips in. He never bothers to inform John his visit ahead of time so John is often surprised to find the talking cat curled on his lap. Some Gryffindors try to rub him or scratch him under his ears but he never lets anyone touch his soft fur except John. John is more of a dog person but he pets Sherlock anyway and tells him amusedly how his cat shape suits his capricious character. Sherlock always leaves Gryffindor tower before midnight because his transformation has a time-limit, so John has once commented that he's like Cinderella. When Sherlock asked what he meant, John explained it's a princess's name from Muggle folklore. The next morning, Sherlock came by and told John rather gleefully that he would very much like to see Mycroft try to fit his too fat feet into small grass slippers. Mycroft glared at them both from Slytherin table but John saw at the same time he returned a delicious looking Danish back to a basket.
Night stroll is their shared hobby. Sherlock always does what he wants but John is a bit of a rule-breaker, too. Together, Sherlock transformed and John under Disillusionment Charm, they explore empty colliders and deserted ground after dark. They hardly go into Forbidden Forrest because they don't want to trouble the colony of centaurs but still have interesting times around the lake. Their favourite place is an unused classroom in the first floor. It's located in a quiet part of Hogwarts castle and even ghosts rarely visit the area. Their only company is Mrs Hudson the Portrait and she has agreed to watch out for intruders when two boys are having fun. John suspects she has a very wrong idea of activities they are engaged in in the secluded classroom but he let her think what she likes because it only seems fair to provide her some stories for gossip in exchange of such a favour. What they are actually doing is practicing charms, brewing potion and studying for project. Sherlock even dares to play violin and to John's dismay, people starts to talk that there's somebody playing sad music in middle of night. However, soon there grows a rumour that the invisible violinist is a particularly bad-tempered ghost grieving over the world's stupidity so people should leave it alone. Later Mrs Hudson has told them that it was her who started the rumour so Sherlock and John would not be interrupted by curious teachers. Sherlock actually appreciated her thoughtfulness so since then he takes her requests on violin sonatas.