This confuses me a lot because I'm planning to buy and amiibo, but I'm wondering if that would give me the character also. Do I have to buy the dlc to use the amiibo character or does buying the amiibo give me access to the character?
The behavior in this case is the same as it was in Smash 4. That is, it'll allow you to play against a computer player (CPU) who is playing as the DLC fighter, but you won't be able to play as the DLC fighter yourself.
Here's some old footage I was able to scrounge up. In Smash 4, Mewtwo was DLC, and you can see from the character selection screen, this person hadn't purchased Mewtwo: