A very cool visual novel! It really nails the creepy and mysterious atmosphere, getting you both itching to find out more and dreading what will happen next. Such good setup makes it practically inevitable that the answers would disappoint, at least a litte, but they're by no means bad. It does drag on a bit around the middle, when you're replaying to get all the portraits. Maybe aggressive use of skipping could alleviate that, but it would ruin the atmosphere, which is its biggest strength.
Still, it has a great beginning and a good ending, which is more than enough to make De Profundis a good VN.