The Nei Qualifier
The "Nei Qualifier" is what I call my system of deciding who is a Protector and who is not. It has two parts.
Item one of the Nei Qualifier states that, to be a Protector, one must fight in direct opposition to Dark Force or even against the Profound Darkness itself.
Item Two states that Protectors need not know of the existence of Dark Force or the Profound Darkness as long as they are taking courses of action that will, if successful, foil the Darkness' evil plans.
I developed the "Nei Qualifier" in response to the popular opinion that the heroes of Phantasy Star III do not meet the criteria for being Protectors. Those who hold this opinion say that since the heroes of Phantasy Star III were not fighting to protect Algo or, more specifically, to protect the seal placed upon the Profound Darkness, they cannot qualify.
What I would like to remind those who hold this opinion is that Nei never knew anything of Dark Force, or the Profound Darkness, or the seal. Nei never once fought for anyone on either Palm or Dezo. And yet, she is a Protector.
The same is true of Alys. Alys had heard of Dark Force from Zio, but it is dubious whether or not she thought Dark Force actually existed. She certainly knew nothing of the Profound Darkness or the seal. And, like Nei, she was unconcerned with events removed from her home planet. Yet she is a Protector as well.
If Nei and Alys qualify, then why do the heroes of Phantasy Star III not? Even though they did not know it, the Alisian heroes were fighting against the plans of Dark Force. Sometimes they were used by Dark Force, but they were fighting to restore order and peace to their world, which is the exact opposite of what Dark Force wanted. And, by stopping Dark Force's plans, Rhys, his friends, their descendants, and even the legendary heroes of the Devastation War were working to preserve the seal's integrity. Think about all of the horrific things Dark Forces accomplished in the other Phantasy Star games. The examples of those atrocities make it clear that the Dark Force on Alisa III could have posed a grave threat to Algo's survival if the demon ever found its way back home.
Also, it is interesting to note that, depending upon one's interpretation, characters such as Nero and Alair might also qualify as Protectors under this theory.