Is this another translation? Are you limited solely to having "the reader" or "the readers"? Because I wouldn't write it that way all.
I would say something like "It's important to remember the needs and expectations of your reader." (And in that case, you can use either singular or plural, depending on whether you thinking about the entire opus of your writing or a particular piece at any given time.)
Given more time to think about it, I might change that, but "Remember, the reader is different" still leave me saying "from what?" Are the readers different from each other? Are they different from me, since I'm the only one who reads my diary? (I hope!) And what is it about their being different that matters? Oh yes - it's their needs and expectations. So why not just say that?
If you really must stick to that format, what about somethign like "Remember, your readers will be different from you" assuming that there are many types of things you'll be writing over the course of time.
So, yes, I have changed my mind about singular or plural - I would now advocate for plural, because this person will write many things over the course of time.