While they're obviously going to suspect Flamie (because of what she has done) or Adlet because he's the Saint of nothing and not a particularly noteworthy warrior, I'd actually put my suspicion on Bunny-hime.

She's a liar, through and through.

- She was toying with Adlet pretending to be a handmaiden just to talk to him and be entertained.
- She's either a worse fighter against fiends than the other six, or fakes it while in front of Adlet to get him to become attached to her.
- The whole scene when Goldov shows up, her demeanor changes instantly, becoming a lot less flippant. She's been putting on a show for the others who don't have her absolute loyalty.

It's definitely not because she's an enemy spy. My guess is because she wants to be do something interesting, and be eternally famous. Its one thing to be Bunny-hime, the strongest fighter in the kingdom. It's another to be The Brave and Beautiful Bunny-hime, one of the Six Braves.

She's indulging in selfishness. She's also in the position has a high ranking princess to have her flower brand faked, like a tattoo or whatever. She could have had it for months before they were actually granted, and no one would know because no one would demand a princess reveal her chest outside of her trusted handmaidens.

The viewers know Adlet is above suspicion because we saw the brand granted to him.