The character card states:

Whenever you have to draw adventure cards, you may discard one card of your choice that you do not wish to encounter and draw one more card to replace it, which you must encounter.

The words "draw adventure cards" are plural. Does this mean you can't use if you only draw one adventure card?

Compared to the Fiend Slayer card which states:

If you encounter a space with instructions to draw one or more adventure cards you must draw one more card than required.

Why the distinction? Just want to play it correctly. Thanks.

You can discard the card when you draw a single but you can only do this once per turn.

If you draw two cards you can ditch one of them and draw another one and then youi must encounter both. if you draw one you can choose to ditch this and draw another which you must encounter.