Jin Soo Seung is the grandfather of Seung "Sonny" Joon.
Jin was an explorer who is always off an adventures. His daughter resented him for this and didn't allow him to see Seung until Seung got sick and was in bed for a very long time. During this time, Jin told them that their family was actually Annunaki, a race of aliens that seeded the Earth with human race. Jin drew Seung comics about a boy named Sonny who discovered an ancient artifact with the help of people with different talents, and eventually Seung adopted the nickname Sonny, revering his grandfather as his hero, even though most of his family wouldn't use his last name, believing he was crazy.
Jin founded S.P.I.E.D., or Strange Phenomenon Inspectors - Extraterrestrial Division, which Sonny eventually became a part of.
According to Leena Patel, Jin would often come to the concert hall she worked at and pretend to play the trumpet even though he couldn't. Jin really just wanted access to the library, where he found a coded book and left it for Leena, who figured out that she could translate it. Jin knew that she was one of the people Sonny would need on his mission.
In 2014, Jin wrote Sonny a letter telling that he needed to go to New Zealand right away to activate the artifact. Upon going there, Sonny learned that all of the places in the comic were real. He assembled the team of people he needed on Pacific Run, and they (mainly Nancy Drew) managed to find all of the pieces Jin had hidden, assemble them together and, most importantly, activate the artifact.