I apologize for the lack of personality information and information on his connection to the prism league, here's an updated version:

Personality: Shawn is a person who puts his friends and pokemon before himself as much as possible. He is very protective of the things he treasures, whether it be a person, pokemon, or object. However, Shawn has trouble expressing his true feelings, especially to people he doesn't know, and will often say nothing despite having a good opinion. His pro-life views also give him an odd sense of humor, often finding something funny that others will not. Shawn is against fighting in any way, partly because he hates to see people be hurt, and because he is, deep down, afraid to get hurt himself. All in all, Shawn can be seen as an introvert with a mind of an extravert, he longs for friends, spotlight, trust, and action, but has trouble understanding and obtaining them.

As for his connection to the prism league, He joined during the years of apprenticeship under Shanze. One of Shanze's mottos was: 'The most important thing a person can own is his identity.' Shawn followed this motto and joined the team he most identified with, which was the prism league. The prism league also supported him in his years of travel and when he became part of IRDA. Every now and then the prism league would ask for his medical advice, but as times were peaceful he wasn't bothered too much.

Hopefully this will fulfill what you were looking for.