When last seen, no one other than Lord Pain was seen living there, since, according to him, everyone got mad at him for breaking things too often, and left as a result.
In House of Pain, however, he is seen to have undead servants, suggesting the residents of Level 9 were undead as well.
According to Lord Pain, everything in Level 9 is capable of being broken.
During Everything Breaks, he and Billy destroy everything until nothing is left to break, thus destroying most of Level 9. It is unknown if it was ever rebuilt.