I can't be the only finding this girl suspicious as hell. She's been trapped in Hoshido for years, but unlike Kamui, she obviously remembers her true origins. And yet, instead of facing the same conflicting loyalties as he/she does, the only one she gives a damn about is Kamui, the one person she knows the least. So, if you side with Hoshido, sure, she's game for going back to her abductors. And if you side with Nohr, where she claims to hail from, it's implied her outfit changes, complete with a veil to obscure her face and probably hide her identity.
Even ignoring that bit, there's still the trailer where it appears that Kamui, in dragon form, is trying to strangle her. However, as we catch a close-up of Aqua's pendant in the fall, his/her transformation is shattering, turning Kamui back into a human.
Couple of things Aqua could be up to:
1. She's bitter as hell about her situation and hates both sides, leading to her siding with Kamui in a twisted perception of "Us against the world", where she tries to insist on Kamui striking them all down in vengeance for their messed up lives.
2. Just like Kamui is unique in his/her abilities and was possibly bred for them, she was bred for hers. Only, she was designed to deal with Kamui's dangerous abilites, and she's now carrying out her purpose by sticking as close to him/her as possible and waiting for a moment of weakness to occur.
3. She's not actually who she claims to be. She only claims so to gain access to Hoshido's inner workings through the guise of a "prisoner". When she learns about Kamui, she switches tracks, because what better way to get at the royals than by playing the part of a bargaining chip. When she actually meets Kamui and observes his/her standing with both royal familes, she figures that the best side to be on is the special little child both royal families would do anything for. Whether she's got her own motives or is acting as an agent of Garon is up for debate, though.
Personally, I'm liking the first option best.