You Now Can (And Should) Enable Two-Factor Authorization On Nintendo Online Accounts
Today, Nintendo has instituted an app authenticator for your online Nintendo account, a security measure that makes it far more difficult for other people to access your account without your consent.
To enable it, head to Nintendo's login web portal, then check out the "Sign-in and security" section. From there, if you haven't verified an email address, you will need to do that before two-factor authorization can be enabled.
This makes a requirement that every new log in for your Nintendo account requires another authorization from you besides just the login and password, usually via an app (in Nintendo's case) or a code texted to you.