In the 600th episode of Running Man, Yang Se Chan admitted his frustrations as the new addition.
While yeah, he joined the cast alongside Jeon So Min, but basically, he's there to fill the slot vacated by Kang Gary.
Gary is one of my all-time favorites and yeah, I was kind of rooting for The Monday Couple alongside Song Ji Hyo from episode 11 I think, up until when the production team had to part the two apart. When Gary and Ji Hyo admitted that their relationship is just for television, the love line deteriorated and in some ways, Gary got lost in the shuffle.
It also didn't help that the sports variety format is causing the cast a lot of injuries.
Se Chan's worth should have been his physical attributes but because that stage of Running Man has passed, Se Chan had to double down on his character - which at that time is still nonexistent.
The departure of Lee Kwang Soo helped Se Chan develop his immature persona since he's already Kim Jong Kook's most loyal sidekick, he competes with Haha in terms of KJK's good graces, and he has this "love line" with So Min. Add the fact that he's the young, disrespectful douche to Ji Suk Jin, the dependable little brother to Ji Hyo, and the obedient young gagman to Yu Jae Seok, Se Chan did the most to get out of the hellish situation he's in. In some ways, Kwang Soo has been setting Se Chan up but in some ways, they had to limit his brashness. Up until that point, that has been Kwang Soo's shtick and in some ways, So Min has been filling in on the other things Kwang Soo had stayed away from, character-wise (like the girl-crazy man or a huge chunk of his betrayer persona).
I don't know how he'll grow as a character in Running Man because while he enjoyed his best years these past few years, it's going to be hard for him to surpass the older gagmen members. Maybe the addition of an eighth member could do the trick but the producers aren't as keen to drop a figure now, especially with the burden it brought to the newbies.