@KairenX I think it's less that he needs to prove his sincerity, and more that she needed to prove hers, since she is trying to prevent her own death and get revenge. The last mission proved that she is not just the emperor's puppet, so Diceon knows she's sincere. Even if he isn't telling her the whole truth of his intentions, at the very least the both know they're united in fucking up the emperor and his plans against Diceon's country, based on her previous life and how he's presented himself thus far.