that girl wasnt a ghost. she was a regular girl. if you pay attention to the stories behind her that were told in both b/w and b2/w2, you'd know this. here's what happened:
Twenty years ago, there was a girl who befriended an abra. she had formed such a strong bond with this pokemon that their souls had become linked. however, one day the abra became very ill, and eventually it took a turn for the worse. the girl stayed by its side though. when the abra's sickness got too serious, it had passed on. however, the girl did not die. instead, she was teleported to the dream world. for many years, the girl tried and tried to get back home, never aging or anything. then, one day she encountered a cressellia in the dream world, one who gave her the lunar wing. this wing had enabled her to go between the worlds, briefly appearing in the real world before teleporting back to the dream realm. this was not enough for the girl, so one day she teleported to her old home where she grew up (the strange house), and saw a traveler (the trainer you play as). she gave him/her the wing, and told the trainer to return it to cressellia. the girl lives to this day in the dream world, enjoying an eternal dream come true.