Gut feeling really. It seemed like the author was intent on making us remember 2 things. That 5 years ago the brother entered the tower and never came out leaving behind a last will that Soojun can't remember. And that Groo is mixed child that is 4 years old and nobody knows who her parents are. It felt like the two things were connected somehow...
Well... yes and no? I skim all the... like 300 chapters (I don't remember the exact number) and they still didn't confirm it... but they made it suuuper obvious that she is the daughter of the brother... it frustrated me a little bit so I stopped following. But, yeah they make it obvious in the novel, but still no firm confirmation
I hate that kind of setting, I wish the author will let them know. It will give the author opportunities to develop the story better with characters development and the overall plot. Wasting over 300 chapters hinting at something big like that is in my opinion dumb but I love the story so I better prepare to get annoyed at all the hints lol. Thanks for sharing

Is Groo the daughter of the lost brother? Cuz that seems to be what we're hinting at...