Question: Let's say I wrote the following in an essay.

> 
> Hello, Anna. I love you
> 
> 
> 

Now I want to write something like this: This line I've written was supposedly said by Anna's boyfriend, but Anna's boyfriend didn't actually say it himself. It's a creative work of mine pretending to be Anna's boyfriend.

Note that this question is tagged with "sentence-construction" because I also want to know how I could state this in one neat sentence.

Answer: 
You simply need to change "*by* himself" to "himself" or even leave himself out entirely.

**But Anna's boyfriend didn't say it himself. I said it, pretending to be him.  

But Anna's boyfriend didn't say it. I said it, pretending to be him.**

*did not say this by himself* means that *someone helped him* to say this.

*did not say this himself* means that *he* did not say this.
* I imagine he said, "Anna, ... I love you".
* If I were in his shoes, I would come-right-out and say, "Anna, ... I love you".
* "Anna, ... I love you", would have been the right thing for him to have said.
* He should have said, "Anna, ... I love you", but he didn't.
* If, for example, he had said, "Anna, ... I love you", they would be together today but he missed his opportunity.

These are all examples of how you could refer to another person's motives or actions. Your choice of "voice" would depend on the nuance that you want to convey.