Replying...
Intro. The promise in question was made when Rudeus, after a misunderstanding where he groped Eris on his 10th birthday, was forgiven by her with the condition that they wait five years, until they were both adults, to be intimate. Eris kept this promise until the very end of their journey through the Demon Continent. On the night she decided to leave him to get stronger, she went to his room and initiated intimacy. Rudeus was hesitant, reminding her of their promise, but Eris was serious and convinced him to go through with it. This effectively fulfilled the spirit of the agreement (albeit slightly early, as they were likely around 14/15 at the time, but the intent was that she was no longer a child) and marked a turning point in their relationship. The main issue for Rudeus was not that she broke the promise, but that she left without a proper explanation, only a short note, which led to a major misunderstanding and emotional distress for him.

[Eris Boreas Greyrat]

@Cyrus☆