@nobaka
First you tryed to justify Sylphy omission in reveal herself to Rudy saying: "she can't do it freely. Because, you know, Ariel will get murdered".
I desagree and explain that she can do it, because: 1. She trusts Rudy, 2. Ariel already allowed.
The inteligence and sense of responsability of Rudy is a fact, otherwise he would be dead since demon continent. Eris also would be dead.
The story and the facts REVEALED until this moment advocates in favour of Rudy being a trustable ally and friend, and it mostly likely it is the knowledge of Sylphy about him now.
If you, reader outside of story, have more information than the characters and even is considering future events... you are in mistake to justify her actions with what she doesn't even know, and please, keep it to yourself and dont spoil the plot to manga readers.
BTW, now i guess Sylphy is not acting wrong, but not because that reasons. No, keeping her disguise aggainst Rudy it is still nonsense. But it is 100% female logic, i have to admit. So it is not abnormal. The teenage thing is also included in this logic.