in oneshot there was just misunderstanding between 2 mcs, that separate them. in series - they all knew each other, and they made some vows as 5 years choldren to marry each other, made some lock and key to find each other, and make some key to other girls to be gatekeeper of sorts for the mane couple, and MC is looking the RIGHT key to open the lock, bcs all this years he didnt even remembered that they knew each other (right till the end) but he remembered their promise!!!! and not only this sound absurd and forced reverse cinderella story of sorts - for almost 200 chapters of 230 author didnt even bother to write about this plot, he is just introducing new girl, he falling for the mc, then they foung out, that she has a key, but then forget about it for another 30 chapters, then in the end author is like "wow, i totally forgot about main plot" and story is like rushing through last chapters: "wrong key, not you, wrong key, not you, wrong key, not you, OH HELLO THERE GENERAL KENOBI, ITS YOU BUT IM NOT GONNA CHOSE YOU BCS AFTER 10 YEARS OF SEARCHING I FALL IN LOVE WITH OTHER GIRL AND I DONT CARE THAT ITS REALLY YOU WHOM I PROPOSED AS 5 YEARS OLD KID"