Jin "Mystic" Seong-jun originally was a substitute for Chunnam Techno University but he never played for them and left them sometime before the end of 2013. He then joined the Jin Air Green Wings Stealths as an AD carry that same year. In Champions Winter 2013-2014, the Stealths failed to make it past the group stage. Eventually, Mystic moved from the Stealths to the Jin Air Green Wings Falcons. He played with the Falcons in HOT6iX Champions Spring 2014 but they failed to win a game and missed playoffs. In May 2014, he left Jin Air, and in November he joined Team WE Academy, where he temporarily switched to jungle from AD carry. He played with them in the 2015 LPL Spring Promotion and they managed to qualify for the 2015 LPL Spring Regular Season after beating Gamtee 2-1.
Mystic joined Team WE as a substitute in January of 2015. He was given the starting position for the IEM Season IX - World Championship in March of 2015. Due to their 1st place finish at IEM Shenzhen, Team WE were invited to compete at the IEM Season IX - World Championship. After a Round 1 loss against Team SoloMid, the team went on to beat Gambit Gaming in Round 1 of the losers bracket. Round 2 of the losers bracket saw WE beat CJ Entus, securing the team a place in the bracket stage. Mystic and Team WE then pulled off the upset of the tournament, emerging victorious in their semifinal matchup over the previously unbeaten GE Tigers. They went on to finish in 2nd place after losing to Team SoloMid in the final of the tournament. Mystic came back to China to finish up the 2015 Demacia Cup Spring Season as a permanent replacement to Styz, the previous AD carry for WE. Two weeks later, they finished up their 2015 Demacia Cup Spring Season with a 3-0 loss to EDG and a 3-2 loss to LSPL team, Qiao Gu, to finally end in fourth overall. They made a late surge near the end of the 2015 LPL Spring Regular Season and overtook Masters 3 to qualify for the 2015 LPL Spring Playoffs as the eighth seed. They matched up against EDG in the first round and with EDG's mid laner, PawN, being out sick, it seemed as though they were going to take the series. However, PawN came back to play game 5 and EDG knocked Team WE out 3-2. Shortly afterwards, WE played in the International Esports Tournament 2015 and managed to make it to the finals but lost 2-1 there to Invictus Gaming. WE participated in the 2015 Demacia Cup Summer Season and made it to the quarterfinals but they lost 3-0 there to EDG. They ended in eleventh place in the 2015 LPL Summer Regular Season and were forced to play in the 2016 LPL Spring Promotion Tournament. They requalified for the 2016 LPL Spring Season with a decisive 3-0 victory over Unlimited Potential.