I've found two of the hidden achievements I was missing, and I'm about to move onto the last one I needed. In the Flirt boyfriend route, you need to max out the Sexiness stat. For the prince, you need to max out the intelligence stat. Following the same trend, the Bad Boy boyfriend's equivalent achievement most likely requires you to max out the attitude stat.
These are all bad endings, which is super interesting. I think playing on Easy mode prevents the scenes that maxing out the specific stats trigger, so you'll need to play on Normal or Hard.
Edit: For the bad boy's hidden achievement, you actually need to max out the Anger stat.
Second Edit: By achievements, I mean the ones in the Steam version. I forgot to mention that.