It's – no, it's not quite alien, it's in some sense an integral part of Sayshen's mind. But it can be distinguished, structurally; it's not human. It's probably the source of the faint tiny hint of bright-blue/ringing-steel that makes the Companion's mindvoice and thoughts feel subtly inhuman. There's a rigidity to it. Something that could bear arbitrary weight without bending, much less crumpling–
–if that force were to come from the expected direction. It feels vaguely like a kind of glass, incredibly strong from one angle but fragile and brittle from another. And, far from all of the strain she can sense in Sayshen's mind is coming from the intended angle.
If Bella looks closer, she'll have a sense of: clear bright line - our sacred trust - foundation built on love. It's not quite those words, more the wordless emotions behind them, a sense of certainty, almost religious faith.
...If Bella looks closely enough, she'll notice that the structure seems to have two parts. They're interlinked, both have the same unnatural sharpness and solidness, but one draws more on the clarity, the other more on the love.