Global variable question

The question is simple. I want to start working with a Global Session Variable.
Where will the data of this variable be stored if the game is over the network?
Is this variable stored by the player in his computer, and he can replace its data? Or is this variable stored only on the server while the player is online, and any manipulations with hacking the client will not make it possible to change it?

It is only about hacking a client.
I want to implement the storage of data about the character during the rebirth, recording his characteristics. Strength, agility, stamina and more. If a player has access to this variable, then he will definitely be able to play unfairly. Therefore, I need to know exactly where the global session variable is stored

Should I use this type of variable to implement the storage of important data about the player?
Level of leveling in any skill, etc.