Somebody who appears to look dull, uninteresting, or could otherwise be described as a 'Plain Jane'. The popular name Sarah is very common and therefore can be used for anybody who is particularly unmemorable or whose name you have forgotten - there being a relatively high chance of it, in fact, being their real name. If she/he has a very plain unmemorable appearance then she can also be described as a Sarah Face.

Literally everyone, ever. Including the writer of this definition.