Urkel wasn't in the first 3 episodes of Family Matters and missed 3 of the next 6.

Topanga missed the first 3 episodes of Boy Meets World and 6 of the first 7. She wasn't supposed to be a regular and the actress who was originally cast was replaced shortly before it was shot.

The Janitor from Scrubs, Castiel in Supernatural, Gunther from Friends, Andy from The Office, and Andy from Parks & Rec are other examples. Maybe they won't be back immediately but there are lots of examples of guest cast becoming supporting or regular cast.