Part of the furniture
Definition 1 of 2
Pro Tip 1/3Work Environment
Often used to describe people or objects that blend into the workplace due to long-term presence. 
With his 30 years at the company, John has become part of the furniture.
Pro Tip 2/3Not Invisible
The phrase doesn't imply someone is unimportant, just that they're a permanent, familiar presence. 
Even though she's part of the furniture, her contributions are vital to our team.
Pro Tip 3/3Neutral Connotation
It is neutral, neither good nor bad, reflecting an established presence without implying merit or lack thereof. 
He's part of the furniture here; you'll get used to seeing him every day.