Jenny Piccolo was brought 'on camera' on "Happy Days" in 1980, (season 8, just after Richie and Ralph left). Cathy Silvers left the show after season 10, and in the final season, Jenny was back to being an 'off camera' character(she'd run off with a Marine!)
'Are You Being Served?' had a number of offstage characters. The producers dropped actor Kenneth Waller, who had played the second('Old')Mr. Grace, following the departure of Harold Bennett. For the last couple of seasons, Old Mr. Grace became an offstage character, and Mr. Rumbold (Nicholas Smith) got a new secretary who became a regular, and took over the 'eye candy' role from Mr. Grace's various secretaries.
And 'Mr. Slocombe' was mentioned in passing only a couple of times, although he appeared on the spinoff, 'Grace and Favour', AKA 'Are You Being Served? Again!' Mrs. Slocombe's best friend, Mrs. Axleby, did not appear, although she was mentioned so often, it seemed like she was a regular! Also, if Mr. Lucas's stories are to be believed, he lived at home with his old crippled mother, in Highgate.. She was mentioned in some of Lucas's elaborate excuses for being late.