You can find a save room or a tv room on the map. They are essentially the same except a save room you can save in. Other than that they are the same. You use the tv to switch personalities and create blood vials. And there library in the same room if you are having any trouble aswell. So you can read about new enemies for example. And how to defeat those new enemies or just about anything in the game.
You use the blood you get from killing heaven smiles to create blood vials. Then you use those vials to upgrade the abilities for a personality.
You can upgrade power,speed,waver or criticals.
Upgrading power gives higher attack power, same with speed.
Upgrading waver makes target locking better.
Upgrading criticals im not sure what it does but it says instant kill.
The highest level of upgrade is level 5.
All personalities have those upgradable abilites. Except for 1 personality which is a personality you use when you die. You get switched to him to go pick up your dead body and then resurrect it.
Also every personality has a unique weapon and ability. You sometimes use those abilities to complete puzzles. As for the fast gameplay in the first video, it was because i was using a personality that runs fast. His special ability is also running super fast but he runs pretty fast even without it.