So my theory is; last chapter or some chapters ago can’t remember, a girl had the same red string connecting her to yeon-ah but it disappeared after he resolved the tension between them so maybe his destined one was her and not sunbae. the required things for the quest to end was believe and end up with your destiny right and yeon-ah mentioned one of those might not be fulfilled so I don’t think sunbae is his destiny , or it could just be that one of them still doesn’t believe in destiny
So my theory is; last chapter or some chapters ago can’t remember, a girl had the same red string connecting her to yeon-ah but it disappeared after he resolved the tension between them so maybe his destined one was her and not sunbae. the required things for the quest to end was believe and end up with your destiny right and yeon-ah mentioned one of those might not be fulfilled so I don’t think sunbae is his destiny , or it could just be that one of them still doesn’t believe in destiny