
To be honest I don't really like Silphy . She immediately thought about herself when Rudeus said that he's looking for his old friend and that was Eris. And also Rudeus kinda forgot about Eris and that makes me sad I hope he's gonna find her on this journey with Silphy

I know you a Eris fan bud but let's think this thoroughly. Sylphy doesn't know the existence of Eris because from the day Rudeus left Sylphy, Rudeus was cut-off from having contact with Sylphy. This means Sylphy doesn't know the existence of Eris and she only knows that Rudeus is working for someone. And what are Rudeus and Sylphy, yes "childhood friends". So Sylphy would think that its her because she never knew anyone who is friends with Rudy other than her.
Second, heart break is just that buddy. Thinking about someone who broke your heart would affect the healing process. That's why they say "Forget about him/her and move on". It's not that Rudy forgot our lovely red-head waifu Eris. It's just that he just doesn't want to think about it cause it will hurt him more. He's just taking a breather from the heart break. He would think about her from time-to-time but not dwell on it much.
Lastly, Eris is training somewhere that is a place only for swordsmen which would be a rural place hidden far away from society (from the looks of it cause its mountainous). They can't just meet her under normal circumstances.

no the numbers are what i got from reading Roxy's spinoff manga. Roxy says she's Rudy's 2nd wife and Eris is the 1st wife and Sylphy the 3rd. I only read like the first chapter or two because i didn't want too many spoilers so i dunno why that's the order, I can just make guesses until i find out for sure.
PLEEASEEE THE NEW CHAPTER IS SO ADORABLE ε=ε=(ノ≧∇≦)ノ