Becky and Darlene’s high school sweethearts were introduced in Seasons 3 and 4, respectively, and remained fixtures for the duration of the series’ original nine-season run.

Becky’s relationship with Mark was a source of great tension, as the straight-A student dropped out of high school and eloped with her dimwitted boyfriend — a decision that continues to haunt Becky on "The Conners".

Darlene and David’s on-again/off-again relationship eventually played as pivotal a role on Roseanne and Dan’s marriage, which made it especially hard to accept that the former lovebirds didn’t live happily ever after once the sitcom resumed in 2018.

https://tvline.com/lists/best-tv-characters-late-cast-additions-seinfeld-buffy-the-office/best-tv-characters-late-cast-additions-mark-david-roseanne/

Roseanne was one of the few sitcoms that added characters in later seasons and they actually worked. They fit in perfectly and were not seen as some lame attempt to create new story lines. The character Sandra Bernhardt played was really good too.

Agree!