>>599The original series premise is normal couple Shinobu and Ataru being accosted by "Urusei Yatsura" (obnoxious aliens), which then shifted to having Lum being more prominent but still as a foil for the couple. So the early stakes are how Ataru can reuinite with Shinobu. That's why the first OP is so heavy in focusing on this.
After that however, the series is retooled with Lum as the main heroine. Shinobu is now clearly not dating Ataru, but she's still in the series, often as one of the four main characters (with the addition of Mendo). Rumiko clearly doesn't want to ditch her, but she also doesn't know what to do with her.
Meanwhile if the anime for the first two seasons wants to fill time, they often give Shinobu or Mendo something to do. Shinobu has a lot of scenes where she's still scheming or seething at Lum, which is mostly gone in the manga.