Ethan left the hideout where he had held Amelia captive, and the first and only person he could see was the violet-haired girl named Eleanor.
She was in this with him. This place was suggested by her.
When Ethan received the Divine Decree, the person who approached him was Eleanor, asking if he was prepared to fulfil his duty as the Emissary of Light.
Naturally, he didn't reject it. Someone who hates Ambrobates down to his core and detests them to death, he had to fight against those who sided with evil.
However, going against the evil-worshiper wasn't his duty, given there were risks involved. He was commanded to take someone crucial into captivity.
Amelia is the only person within Ethan's reach who can play a vital role in weakening Richard once he returns.