A secret you can get but only if you played the demo 1st and get your rewards!
To get this extra reward, you need to play the demo, save it (the save option is right at the very end of the demo) and you collect your rewards when you play Danganronpa V3: Killing Harmony. It's ... an extra scene in Chapter 1 with Tsumugi and a sexy mysterious Ultimate.
What to Do...
Well ... It's an extra scene in chapter 1 which you can only get if you have the nail brush - talk to the Ultimate Cosplayer Tsumugi the morning after the failed attempt and click yes as an option when you talk. When you come to the free play event, chose to rest in your room (you may e able to achieve this cut scene if you still choose to see a friend but I chose to stay) and you will then go on to a nice pedicure scene not just with her cuteness but with a sexy hunk who sadly will shortly be murdered? fail? Please save the game beore you talk to Tsumugi to ensure everything goes well xxx.
Hope you enjoy!
Some Sweet Screenshots
I used spoiler just in case you didn't want to see them :)