Let's see I'm probably speaking out of my ass but Sya transported him to that place...btw ...

Let's see
I'm probably speaking out of my ass but Sya transported him to that place...btw this arc is about how he got the heart for his bride(if that isn't obvious).
Frederick gets into the household as a servant to get an inside information of what is going on in the household kinda trying to figure out why he had to have a shitty childhood.
My understanding is that Eric and Frederick had the same mom. The mom was prob a commoner and when Eric's dad died she got married to the earl and gave birth to Frederick. Frederick was supposed to have his father's title passed down to him but when the mom either died or divorced his dad, the dad got married to a lady that inherited her father's marquis title...The lady gave birth to a child and because of the new law that pretty much states that Frederick is illegitimate. He got cut off from inheriting anything from the dad.

In this sense they share similar situation because when they were younger/ when his mom got married to Frederick's dad, Eric got shunned and bullied and possibly raped when people found out that he wasn't a noble despite the fact that he was living in a noble's house. Frederick was a snotty kid with a superiority complex and the mom encouraged that behaviour. She was playing favorite.

In present day, his dad supposedly hired an assasin to kill him but so as to not create a scandal or something. the person he assigned it to was sloppy asf so he didn't suceed, Frederick finds it funny that a hitman had the gall to pretty much walk up to his house in broad day light and try to kill him.
Eric tells him that he had been in similar situation before but he fought hard or something of that kind..