Responses
Tl;dr: My theory is that the red string likely represents relations with persons he might have had romantically in his (Yeon's) past life, and the fact that he likely has to choose again, but his choice might be hampered by Kihoon's own anxiety.
Are topics character-limited??? Anyways link to my long-winded spiel (theory) on the past-life plot & two strings situation >> https://www.mangago.zone/home/mangatopic/18862692/