The apartment doesn't have much in it in the way of furniture.
What pieces there are in the apartment were mostly left behind by the previous residents, the only significant piece an antique dresser in the bedroom. In every room, there are a two constants - books and sheets of music. Music papers part of the wall in both the living room and bedroom. There are two guitars in the living room, leaning against the wall, as well as an ancient piano close to the window.
The charcoal drawings and the photos taped behind them hang in the bedroom, around a mattress with sheets on it that serves as a bed. The kitchen is dingy but clean, with a small table and a few chairs, also left behind by the previous owner. The refrigerator is often bare, though sometimes one can find fruit or simple, ready to eat food inside.
In the bathroom, there are a few unusual items mixed in with cheap soap and shaving cream - a straight razor on the sink, a strop hanging from one of the towel racks, and pots of white and black face paint sitting by the faucet.
There is an old laptop sitting on the floor in the bedroom, just about the only piece of modern electronics in the entire apartment. The only other items of note in the bedroom are the gun on the floor and the bullets piled haphazardly next to it, along with a cleaning kit and a long knife.
The apartment feels lived in, but the lack of furniture makes it feel somehow abandoned and empty at the same time. The windows have dark, dusty coverings, again left behind by the previous tenants. It is a fairly clean place, if dusty and dingy by the very nature of being in The Hamartia. All in all, there is the sense that the person in the apartment does not do much living there, and that their needs, such as they are, are geared toward anything but average, immediate worldly concerns.
Edit: There is now a roommate living in the living room of the apartment, and she has done an excellent job of making it feel more like a home. Jack has moved all of his things into the bedroom with the exception of the piano, which simply didn't fit, and is still hovering amongst car parts and welding tools.