Sorry. We don't know the name of the narrator. And neither do you! The only name she gives us is Mrs. de Winter, a name she acquires after marrying the wealthy Maxim de Winter. She doesn't seem to be writing her story down, or telling it to somebody. Instead, she's remembering it, an unspecified number of years later. Either way, it's told in the first person: we don't hear about what Mrs. de Winter did, we hear about what "I" did.
Many of the things our narrator remembers would crush her identity as Mrs. de Winter if they were made public. She definitely can't talk to Maxim about this stuff. Her mind is the only safe place for her story. So, although memory is never flawless, we can trust that we are getting the version of the events exactly as she remembers them. She's doesn't have any motive for holding back information because her audience is herself.
The frankness of Mrs. de Winter's narrative allows us to see her biases. She is never looking at either Maxim or Rebecca through objective eyes, nor does she pretend to be. In her mind, Rebecca is always a villain, and Maxim is always the victim. (Yep, even when he admits that he's a murderer.) Maxim, Frank, Mrs. Danvers, Jack Favell – all these characters are seen through the filter of her eyes and her imagination.
So much of Rebecca consists of the narrator's imaginings of what might happen or what others might be saying or thinking. In terms of Rebecca, the problem is magnified. Everything the narrator tells us about Rebecca is either what she imagines, or what she hears from other characters, all of whom have their own motives and their own imaginations guiding their versions. After all, our narrator has never actually met the woman, so we can't really trust what she says.
This multilayered narrative technique presents readers with a puzzle that can't ever quite be solved. At the end of Rebecca, we know the many of the details of Rebecca's murder and the subsequent cover up, but we still aren't at all sure what kind of person Rebecca really was or what really happened between her and Maxim.