Ghosts can appear solid or shadow. Not sure why some do and other don’t. I guess we will never know until we cross into that side. Perhaps that person died in that apartment you were in years before you folks took over.
The bible has one incident of a dead person being summoned by a witch. It fits better with a general biblical understanding that this would have been a demonic illusion rather than any actual summoning of a soul. In the Transfiguration, Jesus showed Moses and Elijah (in full form, not ghosts) and this would be credible because one is dealing with God here.
The mainstream Christian understanding, and I now have no real reason to doubt it, is that there aren’t any ghosts. There ARE demons, which are angels that rebelled against God along with Lucifer who became Satan. The love of God will repel demons, which are driven by hate of God.