Ryuji's life certainly took a turn for the unusual with the start of the
series as not only did his cousin Eriko arrive and cause a fair bit of
trouble, but the Lost Precious that she got him to help her hunt turned
out to be a dragon named Rose. Rose is certainly adorable but her
arrival has caused the treasure hunters society to come and bring all
three of them in to figure out exactly what's going on. As we learn, the
majority of research about dragons came from a man whose notes they had
some ten years prior as he was the only one to actually find a dragons
egg and raise it. Rose is the first time the researchers have seen a
live dragon in human form so they desperately want to understand her
inside and out.
Eriko is rather amusing about all of this as the traits she can take on as she matures are listed off, including having a small pair of wings that can sprout on her back at any point soon as she's just about at the right age. All of the discussion about this brings back some memories for Ryuji to a time ten years prior when his parents took him to Albania and he has hazy memories of an egg hatching and the sensation of it as he touches Roses' back in search for potential winged-ness. The key thing that comes out of this is that Rose may have imprinted with Ryuji and that explains their relationship, though it could have gone back to when they first met ten years prior. It certainly adds an amusing twist to that childhood friend of sorts who becomes something more when they're older but that's not really the angle that the show is playing, at least now yet.