The concept is as follows:
Lisa is believed, by many, to be a manifestation/ghost/instrument of Valtiel, etc. To make this possible, she would have had to die before the events of the game. Most assume this happened, and further, that she was specifically murdered by Dt. Kaufmann. Though some information exists to suggest this as possible, there is no proof.
But, if she's real, that introduces issues. She's confined to the hospital. She only appears in the -otherside-. Eventually, she 'dies', and takes on mannerisms not unlike the possessed nurses and doctors seen in Alchemilla Hospital. How could this all be true if she's a real person and still alive?
The answer is: The Virun VII Crest used to seal Alchemilla Hospital into the -otherside- was placed directly on her person.
Reason:
The best explanation for this is that Alessa is going to seal off Alchemilla anyway, and so uses Lisa as the focus to prevent her from succumbing to the parasites which have taken over the rest of the hospital staff. Perhaps Lisa's being drawn into the Otherworld was unintentional or unavoidable, and Alessa means for everyone within to not survive what she plans to do. This could be a means to allow her 'angel' to retain her humanity until it's over, and avoid the fate of her contemporaries.
Evidence:
- The Crest is visible nowhere in the hospital.
Unique among all [otherside] locations in the game, there is no visible evidence of the Crest. Everywhere else, in the school, the Antique shop, the sewer, and all around town after it falls into that abyss, the Virun VII Crest is worked into the environment, a watermark of the power which was responsible for creating what Harry sees around him. Alchemilla is an exception to this. Although it looks every bit as hellish as the aforementioned locations, the Crest is conspicuous in its absence. - Lisa only appears in the [otherside].
This is another fact easily explained by this theory: If she bears the Crest, it would make perfect sense that she is never seen in the normalside version of the hospital. No other character is limited in this fashion. If she were merely a manifestation, this would not apply to her, because other manifestations do appear in the normalside town. There is no other theory which adequately explains this limitation. - Lisa cannot leave the Hospital.
If she bore the crest which sealed away the Hospital, it would explain her otherwise-unexplained inability to leave the Hospital. She feels she is 'not supposed to leave this place', which would make sense if she bore the Crest for this location. She would be compelled to stay, regardless of what she might otherwise want to do. - Nowhere and Lisa's Death
Lisa claims to finally understand what's going on, and that she's the 'same as them', them being the other nurses. This realization is considered by most to trigger the disintegration of her manifestation and revert her into just a regular monster.
However, there's an alternative to it merely being the result of self-realization.
The Flauros is used to attack Alessa and the result of this is that the [otherside] begins to decay. Everyone ends up in a place called Nowhere, which seems to contain no spatial continuity. The [otherside] is breaking down, and if the Flauros does this by somehow weakening or destroying the Virun VII Crests which sealed places into it, and Lisa bears the Virun VII Crest for Alchemilla, then we can attribute her rapid disintegration to the effects of the Flauros.
She begins exhibiting strange behavior immediately. Soon, she begins to physically break down, and by the end, she's little more than mindless, with just enough left to identify one of her tormentors and exact revenge upon him. She is, in other worlds, reflecting the state of the [otherside]. - Lisa isn't quite "the same as them".
It is revealed (see below) that Lisa comes to understand that she's the same as the possessed doctors and nurses haunting the hospital. Most people assume this means that Lisa is a manifestation, because they believe the nurses and doctors are manifestations. This is not correct.
Lost Memories, pp. 26-27, indicate that the Nurses (and by extension, the doctors) were real people drawn into the Otherworld and attacked by a parasite which assumed control over their bodies. We are given extra reason to believe they were real, because we witness it happen to Cybil (the same exact terminology was used to describe her attack by the parasite) and, if we save her from the parasite, we learn that it is not fatal. Cybil emerges from the experience intact. This means, if Lisa is the 'same as them', it also means she's a real person and not a manifestation or ghost.
If one factors in the idea that she bears the Virun VII Crest, it may explain how she can be a real human being and yet remain unaffected by the parasites. It is the easiest explanation which accounts for both facts simultaneously. - The Virun VII Crest is explicitly associated with Lisa in Silent Hill 3.
Except for the 'worthless' physical object called the Seal of Metatron, the Crest itself appears only once in Silent Hill 3, fittingly, as the normalside version of Brookhaven is transforming into the [otherside]. Examining this Crest gives Heather a flashback of Lisa, a short rumination on her behalf, and the realization that this Crest does not make her head hurt, as does the Halo of the Sun seen frequently throughout the game. It is never seen again, and Lisa is never mentioned afterwards.