What word would describe someone who doesn't generally leave much of an impression on people?
I'd suggest such a person would be called a nonentity.
A person who is easily overlooked is often called a "wallflower", although that term has other connotations as well.
A number of years ago I came across this book The Lexicon of Intentionally Ambiguous Recommendations: Positive-Sounding References for People Who Can't Manage Their Own Sock Drawers
Depending on your needs, it could be useful, e.g.
- The impression s/he conveys to others is no act
- He was with our firm a few years back, but I can't remember the dates he worked for us
- I can't remember ever hearing a colleague complain about her work
- I can't begin to tell you what a fine person she is.
To be more direct, perhaps I could suggest: