The following are characters mentioned in passing.
Dunder Mifflin Scranton
Catherine
Catherine was a former employee of Dunder Mifflin Scranton and was mentioned in the episode Sexual Harassment. Michael claims that she was hot and that she wanted him. Kevin, however, replies that "she wasn't that hot."
Marjorie
Marjorie was mentioned in the episode The Fire as a supposed employee of Dunder Mifflin Scranton. Dwight explains that Marjorie is not there that day. In the current show canon, it is likely that Marjorie does not actually exist at all due to the fact that the supporting cast was not fully established when The Fire first aired. Many fans had initally linked her to possibly being Luanne before show creator Greg Daniels explained who Luanne was.
Tom Peets
Tom Peets was a former accountant at Dunder Mifflin Scranton. In the episode Performance Review, Michael reads some old suggestions from the suggestion box. One of them stated "We need better outreach for employees fighting depression" and it was signed by Tom. Michael thinks this is a fake suggestion considering there is nobody named Tom, but Phyllis reminds him that Tom had shot himself.
In a press still from Did I Stutter?, Tom's last name is revealed as Peets.
Steve
Steve is the vending machine stocker at Dunder Mifflin Scranton. In the episode Booze Cruise, Jim is helped by Steve to pull a prank on Dwight by putting all of Dwight's belongings in the vending machine. Dwight mentions Jim being friemds with Steve.
Miles
Miles was mentioned in the episode The Carpet. He was a former member of Dunder Mifflin Scranton but quit to start up his own business.
Urkel Grue
Mysterious German woman that Michael met and was photographed with in Jamaica in the episode Back From Vacation. She was, in, fact Jan Levinson, but Michael made up the name for cover. Jim later suggests her name when Michael tries to explain the Scranton branch the identify of the woman in the photo he accidentally mass emailed to everybody.
Randall
In the episode Sexual Harassment, Todd Packer revealed to Michael that the former CFO, Randall resigned due to sexual harassment charges regarding sleeping with his secretary.
This explains why, just a few episodes later (Valentine's Day), Michael must meet with the new CFO, David.
Other branches
Denise Dimm
Mentioned in the deleted scenes of Launch Party, Denise Dimm is the head of the party planning committee at the Buffalo branch. As a result, Angela considers her her rival.
Jeff
In the episode Survivor Man, Toby mentions having attended a wilderness retreat with Jeff from Albany, suggesting that he is the new regional manager of the Albany branch, replacing Craig. On the other hand, Jeff might hold some other position since Toby's presence at the retreat indicates that is was not exclusive to regional managers.
Mark Chisholm
Another attendee of the above wilderness retreat was Mark Chisholm. Considering Ryan only invited regional managers (other than Toby), he must be the regional manager of either Rochester, Nashua, Akron, Camden, or Yonkers.