My friend let me borrow his account to play all his downloaded switch games, and I've been playing a lot of Fire Emblem on his account. Now I want to buy the game. If I buy the game and play it in my own account, will it keep the data from playing on his account since all the data is in the Switch itself?
Unfortunately, you'll have to start a new game.
According to Nintendo Support, you can transfer a user (including save games linked to that user) from one Nintendo Switch to another.
- Save data cannot be merged or transferred between users.
Since you played on your friend's account, you can only transfer the save if you also transfer his account.
Which would mean...
- Once the process is complete, the user information, the associated save data, and the software purchased with the user account that is transferred will no longer be available on the source system.
... that his account (and all his saves) will be missing from his Switch. I don't know your friend, but I don't think he'll be very happy about that.
For future reference:
Don't use your friend's account to play games, create your own account and use that one. You can still play the games he has purchased, as long as you are playing on his "primary Switch".
Unfortunately, once you've transferred your own account to your own Switch, you won't be able to do it again:
- A Nintendo Account must be linked to the user account you wish to transfer from the source system. That same Nintendo Account cannot be linked to a user account on the target system prior to the transfer.
This means you can't transfer an account to a Switch that already contains that same account. You could in theory create a new account for every time you want to play on your friend's Switch and transfer the data to your own, but...
- The target system cannot have more than seven users at the time of the migration (there must be room for one more user on the target system).
A Switch can only contain 8 accounts at any one time. Sooner or later you'd have to sacrifice an account (and its saves), to make way for a new one. Or you'd have to buy another Switch. Neither is a great idea.
You could, however, link your friend's Nintendo account to an account created on your Switch, and register it as your friend's "primary Switch" to access your friend's games. However, since an account can only have 1 "primary Switch", he'd have to switch back (no pun intended) to play any digital games on his own Switch.
Another alternative, is to have a subscription to Nintendo Switch Online, which offers cloud saves. This would allow you to play games on your own account on your friend's Switch, backup the saves to the cloud, and download them from the cloud to your own Switch. Note that not all games support cloud saves, such as Splatoon 2, Pokémon: Let's Go, and Pokémon Sword/Shield.