I think it might be a love story with a dark and gritty edge. Maybe the characters face life-threatening situations or have to make tough choices that put their love to the test, all within the context of the osu world.
The Vanitas no Carte manga has come to an end. The creators completed the story and provided closure for the fans. It was a memorable journey throughout.
If 'osu my first story' is about the osu! game, perhaps it's about the initial fascination. When starting osu!, the unique rhythm - based gameplay can be really captivating. The first time seeing those circles come at you and trying to click them in time is quite an experience. It could also be about the first time making friends through the osu! community, sharing strategies and having friendly competitions.
I'm not really familiar with the specific story behind the red X in osu. It might be tied to a scoring system, a level completion marker, or something else game-specific. You might want to check the game's documentation or forums for a more detailed explanation.