Sanae Higashihongan

She's the ghost living in Koutarou's apartment.

It is later revealed that she is not an actual ghost per se but some leftover spiritual energy in room 106 before her real body got admitted to the hospital. This would also be the reason why she is freely able to change her clothes but only to limited sets since the real her has only seen these clothes before going into a coma.

Being not the actual person, she doesn't share some things with the real person such as physical appearance and memories. As such, her real body has aged normally and have green eyes instead of blue, and long, orange hair instead of violet.