I Tried to Become a Worthless Scumbag to Break Our Engagement for His Happiness, but Instead I Was Hugged, Kissed, and Sworn Eternal Love by the Ice Knight (BL)

The ever-optimistic Fleck, who adored Heliosphere, suddenly realized: Heliosphere has to marry someone like me because of my strange constitution—how terribly, terribly unlucky for him.

To spend his whole life with someone talentless like me—what misfortune.
This can’t do.
With that in mind, Fleck played the part of a worthless scoundrel, and after much struggle, somehow managed to get their engagement annulled…

And so, sent to the harsh Eastern Demonfront to “correct” himself, Fleck worked desperately as a civil officer. In the spring of his sixteenth year, at last, the Kingdom of Kamiokande returned to the royal capital.
Steeped in the common sense of the Eastern Demonfront, Fleck thought that the happy ending he had dreamed of must have already come to Heliosphere.

When they reunited, Heliosphere—still a student—had become a knight of ice.
Sky-blue eyes, shining silver hair, tall and knightly, he had grown into a breathtakingly handsome young man.

There was no way such a splendid Heliosphere could be without a fiancée.
If he wasn’t, then surely the world had gone mad.
Fleck was certain he must be engaged to some wonderful young lady… a saint, surely.

And yet, he wasn’t.
Instead, he asked Fleck to help with student council work.
Heliosphere was simply being kind to him—comforting a civil officer who had endured the front lines alongside knights. Nothing more, nothing else. Fleck desperately tried not to misunderstand.

And yet, Heliosphere hadn’t changed.
Not one bit.
Neither had Fleck.
His appearance might be worn down in many ways, but his heart held only one wish: Heliosphere’s happiness.

Though he had given up on school life, Fleck now spent his days with the very person whose engagement he had annulled, working by his side.
Day after day, he suppressed his feelings, even as the kind knight Heliosphere swept him into princess carries, bought him clothes, and spoiled him. And still, Fleck fought on, striving to secure Heliosphere’s happiness.

This is a light, fluffy isekai fantasy.
Same-s3x marriage is the norm in this world.
The protagonist is “incomplete” in many ways, but since he’s steeped in the Demonfront’s common sense, he doesn’t feel the seriousness of it.