He's the prince Rin saved with the soma and Rin saved his fiancee with the potions she made. Glay's fiancee is also an adventurer and they form a party with Rin and Earhart later on.
I avoided the 1st story bc I read about the two bros where the big bro burned his little bro's eye and the one about Koganei. I thought the 1st story was going to be twisted too but it turns out its really a sweet story and I keep rereading the first story.
Some things about Glay
He's the prince Rin saved with the soma and Rin saved his fiancee with the potions she made. Glay's fiancee is also an adventurer and they form a party with Rin and Earhart later on.