Like Derek doesn't even know Arthur or his family exists. He has recently remembered his lost memories of his parents, but the only extended family he knows about is that his mom had a sister. Since he only recently remembered it, he hasn't taken the time to seek out any biological family. And he's not one to openly reveal his actual name he was born with. Like Tatsuo knows, and that's it.

I'm down for arthur looking into it on his own, and i'd be down to hash out what he probably would have found. Because one advantage Arthur would have had that Derek didn't: names of people and pictures. Derek had literally nothing to go off of for years.