Katrina's bedroom is in the West Wing of the house on the second floor. Like most bedrooms here, it offers not only the necessary but an en suite bathroom and walk in wardrobe. Katrina tends to play loud music, causing Alvin and Evanna, tired of sleepless nights, cast a Muffliato charm on the door.
As far as she is concerned, this is Katrina's private space, and she has a habit of locking her door magically to keep out her brothers, and is on the verge of putting a password on it.
It is themed pink and purple, or the bedroom anyway; the bathroom and walk in wardrobe are beautifully simple. The windows are charmed to show a different landscape each day for Katrina to sketch, paint etc.
She keeps all things that matter in a secret compartment she found, and as far as she knows she is the only one who knows about it. It's a vault, one of those fingerprint ones like the ones at Gringotts. As to why it is there she has no idea, but likes taking advantage of it anyhow.
Like she prefers not to have intruders as she despises them. She has even gone as far as to put an intruder charm on the door. It is no longer a surprise to the elves as it used to be and they can easily bypass it, as can Alvin and Evanna as they are the rightful owners of the mansion.