All psionic classes has own key ability except Aegis. I searched through many manuals but I found nothing to help me. I ve found "Table: Ability Modifiers and Bonus Power Points" but Aegis can't use and "A saving throw against your power has a DC 10 + the level of the power + your key ability modifier (Intelligence for a psion, Wisdom for a psychic warrior, or Charisma for a wilder)." Nothing for Aegis. Can you help me understand?
No psionic class description defines a “key ability modifier.” This is not a defined game term or anything; it is a description of a certain ability modifier that manifesters use for saving throw DCs, bonus power points, and minimum requirements to learn and manifest a power. But none of the classes says “the X’s key ability modifier is Y,” they just say that you use Y for DCs, power points, and requirements. The only places you’ll see the term “key ability modifier” are in shared rules like those for bonus power points, or in the collective ability of the tactician and vitalist classes (and various archetypes of other classes).
The aegis is not special in this regard.
However, relative to most psionic classes, the aegis is somewhat special: An aegis is not a manifester. They do not manifest psionic powers, and they don’t use any of the rules for doing so. Their psionic ability is related to their astral suit and the customizations they can create for it.
The aegis does get power points per day, and gets bonus power points for high Intelligence, as a psion would. In this regard, you might call the aegis’s “key ability modifier” Intelligence. And for the purposes of bonus power points, that would be an accurate description of Intelligence for aegides. For other purposes, the term “key ability modifier” is less accurate. Because aegides are not manifesters. For instance, there is no rule in the aegis class description saying that saving throw DCs are based on Intelligence—they don’t force people to make saving throws, so there’s no need to define what the DC for that throw would be.
In addition to “key ability modifier,” bonus power points depend on manifester level. That is a defined game term, far more so than “key ability modifier.” Classes do define how their manifester level is calculated. And since the aegis gets bonus power points per day, they need something that can “fill in” for the manifester level those rules expect. So for the purposes of their power points, the aegis is treated as having a manifester level equal to their class level. But this is just “faking” it for the sake of those points—the aegis is not a manifester, and does not have an actual manifester level.
Anyway, compare and contrast the aegis with the soulknife: the soulknife, like the aegis, is a psionic class that is not a manifester. It doesn’t even get a power point progression! It just gets the Wild Talent feat for 2 power points—which the class never uses. There is no “key ability modifier” for the soulknife, because there is no need to refer to any ability score that way. No bonus power points, no saving throw DC, nothing. So what is a soulknife’s “key ability modifier”? It doesn’t have one. They like Strength or Dexterity, and Constitution, same as any other warrior. Some blade skills rely on Wisdom, but it’s very easy to make a soulknife who doesn’t use any of them.