[@Double]

I've never gotten to play Birth By Sleep but I understood what Cyrus meant by "revealing the true self through battle"

You can tell a lot about a person by the way they move or react. An underhanded person would use underhanded tricks in battle. Someone who is calculating with have a quicker reaction time because they will be prepared for all. People that are brash will be the same in battle. Those that wish to protect will be more likely to rush into battle or be spontaneous which can make them hard to read or make them not fear death if another is able to live.

That is how Bellasiel viewed his speech which is why she didn't step in. Cyrus needs to see who Arthal is in his own way and judge his worth.