There's plenty to discuss these days in the Gryffindor common room with the Apparition test looming large, with Ron and Lavender on the rocks, and with the Horcrux memory yet to be collected.
Kreacher and Dobby report that Malfoy has been using the Room of Requirement, and Harry realizes that Malfoy has also been dipping into the Hogwarts supply of Polyjuice Potion in order to disguise his cronies, Crabbe and Goyle, as little girls so that they can inconspicuously keep a lookout while Draco conducts business in the secret room.
Harry tries several times to open the Room of Requirement (which requires great brainpower and the right frame of mind), but he is unsuccessful.
Moaning Myrtle tells Harry about a boy who has become her friend. This particular boy comes to hang out with her and to cry to her.
Harry discovers Tonks near the Room of Requirement one day. She doesn't look very healthy – she looks stressed and sad. Harry wonders whether she was in love with Sirius and if her strange, distracted behavior and disheveled appearance are a sign of her grief for him.