Moneta is "the most beautiful woman [Kassad] had ever seen" (2.221). Well, he must like them strong and silent: she's in her late twenties, hardly ever speaks, and might not even be human. Kassad doesn't even know her name, calling her Mystery, until she reveals that she can be called either Moneta or Mnemosyne. In Greek mythology, Mnemosyne was a Titan (like Hyperion), the Titan goddess of memory.
And at first, she seems like a memory, or at least something unreal. She comes to Kassad only in the military simulations. They kill people, then make love on the corpse-strewn battlefields. Sexy! Another cadet tells Kassad, "the HTN stuff doesn't simulate, [...] it dreams [...] and what it dreams, it lets us dream with it" (2.240). So she's helping Kassad realize his dreams of war and sex combining into one, "the terrible beauty of combat" (2.292).
That's quite a paradox, but it's something Kassad would believe in whether Moneta existed or not. She just personifies this strange dichotomy. To some people, the act of war can be beautiful, we guess. Like a very violent ballet. Didn't you see Black Swan?
When Moneta rescues Kassad on Hyperion, she's more of a nightmare than a dream. She tells Kassad that she's traveling backward through time with the Time Tombs: "Your past. My future" (2.437). She's able to communicate with Kassad without speaking, "through a medium that was something less than telepathy but something far more sophisticated than [technology]" (2.454). That's all well and good, and Kassad really enjoys it when they start traveling faster than time with the aid of the Shrike and decapitating their enemies with their bare hands.
Then things start getting weird (as if they weren't already). They're having their customary post-battle intercourse, when Moneta starts to morph into the Shrike: "Moneta's lips wither and curl back, revealing rows of steel blades where teeth had been" (2.491). And that's not all. Other places develop teeth too. And spikes. When Lady Gaga sings "Show me your teeth," we don't think she was talking about teeth down there. (Well, with Gaga, who knows?)
Kassad escapes, barely, but this relationship is officially wack. Moneta and the Shrike, one and the same, beauty and the beast. But isn't their relationship a lot like war? War can be fought honorably—but once you take advantage of your enemy and slaughter without fighting a fair fight, then you've gone too far. Do you believe there is such a thing as honorable warfare?