25/21 k-series * Happy or sad ending? *

As painful as it was, the story is much better the way it ended, because ending up apart was the most realistic and meaningful conclusion for their specific character arcs.
While the romantic in me wanted Na Hee-do and Baek Yi-jin to stay together forever, keeping them apart served a brilliant purpose for a few key reasons:

1. It fits the show's core message about youth
Twenty-Five Twenty-One isn't actually a traditional romance drama; it is a coming-of-age story. The central theme of the show is that youth is beautiful, fleeting, and temporary. First loves are incredibly powerful because they shape who we become, but we rarely marry them. If they had ended up together in a perfect, tied-with-a-bow ending, the show would have lost its realistic, bittersweet poetic impact.

2. It preserved their love from rotting
They broke up because the crushing weight of real life—distance, trauma, and demanding careers—was starting to make them resent each other. By breaking up when they did, they were able to save the beautiful memory of their love. If they had forced a long-distance relationship to work while Yi-jin was suffering from severe PTSD in New York, they likely would have grown to hurt or dislike each other. The breakup allowed Na Hee-do to remain his "eternal muse" and savior.

3. It honored Hee-do’s boundaries
Hee-do grew up completely neglected and lonely because her mother always prioritized the news over her family. When Yi-jin started doing the exact same thing, Hee-do had to make a mature, healthy choice for her own mental well-being. Watching her step away from a love that was causing her childhood trauma to repeat was a massive moment of personal growth for her.

The Bittersweet Verdict
They were exactly what they needed to be for each other at that specific moment in time: the right people at the wrong time. Yi-jin gave Hee-do the confidence to become a gold-medalist fencer, and Hee-do gave Yi-jin the strength to keep living when his family lost everything. They saved each other, and that is a beautiful ending, even if it didn't end in marriage.