I skimmed through the previous chapters to make sure, and yes, this is the very first time we've seen this butler in the series. When MC mentioned the perpetrator is someone close to the victim, I thought it would be someone we've seen so far, but this is totally out of left field. Yes, it's understandable to view a servant as someone being 'close' to their employer, especially as an aide or secretary, and yes, the butler being revealed as a perpetrator has been a trope that's been used frequently since the idea was first established, but we know nothing about this guy.
This series isn't even tagged as a comedy to show how 'serious' it is to use such a trope, so this just feels worse than lazy writing to me. I don't even care if they try to do a flashback of this guy's background to better describe his character, the dye is already cast and it's too strong to be overwritten with anything else to make it better.