This is about the Special My Room you can access while in the main story line of EXTRA.
It is a place that is removed from the rest of the game (the main plot) where you can discover many things about allied Servants, such as their past, the trauma they’ve gone through, or even their true name.
You can access the Special My Room by continuing to progress the story, frequently visiting the My Room of allied servants, and fully upgrading SE.RA.PH1.
However, while you may think it a mistake, from the 5th round onwards, the conditions that must be met to raise Cas-ko’s flag (and only for Cas-ko) become very severe.
When talking to her in her My Room, after selecting one of the conversation options if you hear her say “Would you be so kind as to try again from your last save point?” then even if you haven’t saved your game in the last two hours, you should definitely swallow your tears and reload.
- ^ SE.RA.PH completion or fill (as in clearing SE.RA.PH’s storyline or something), upgrade sounds better, but it really depends on what this is called in the game.