I don’t mind giving out spoilers but dont go around crying about it.
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Because of yoohyun. Yoojin and yoohyun bond is as we all knew is intense. No one can break that bond. Yerim wanted the same bond that hyj selflessly given to hyh. She knew she will never receive the same familial bond like that.Even when hyj tells and assured her, deep down, she still thinks its just what hyj said to calm her down. And her elemental spirit doesnt help either lol. She keep telling yerim to take hyj for herself, that both hyj and hyh never wanted her anyway. But yerim being the good girl that we all loves, she finally let herself learned that family can be created,she decided to accept hyh coz she didn’t realise it at first coz she was blinded with her spirit and her insecurities, but the minute she realised, hyh has been accepting her all these while. She’s especially close to hyj eldest son too.

i might be looking into things too closely but i think yerim tries to cope w her loneliness w plushies