Some of the most memorable moments in the series are these.
* Ataru's class is raising caterpillars and, despite an overly large and ugly one eating all of the food, Ataru spends the entire episode working with Lum to keep the rest of the class from hurting it, believing it will become a beautiful butterfly. At the end, it turns out to be a fairy whose first words to Lum and Ataru are, "Mama! Papa! Thank you for believing in me."
* In one season finale, Ataru mistakenly eats some food that Ran made for Rei that increased exponentially in size once inside the stomach, thus making him get incredibly sick. Lum enters an Alice in Wonderland-style realm to look for Ran to get an antidote, but Ataru's monitor flatlines before she does and Lum breaks down sobbing. [[spoiler:Fortunately, Ataru got better and the monitor was simply unplugged on his end.]]
** For this troper, the fact that this episode comes inmediately after the infamous "And There There Were None" episode, where all the Tomobiki gang, including Lum, [[spoiler:put Ataru on the verge of a heart attack as a result of an overly elaborate prank (he got better, of course)]], makes it the more heartbreaking: if we don't give the [[ResetButton reset button]] trope the serious consideration it deserves, [[spoiler:probably Lum has to cope with the fear of fatally losing her Darling twice in a very short period of time]]. For a comedy series, this feels very tough.
* The episode Japanese viewers voted the best one features Ataru breaking down in tears in the middle of a dingy alley, clutching a small doll that Lum left behind when she suddenly left town. [[spoiler:She merely had to return to her home planet to renew her passport so she could continue living on Earth. But, it's the first time Ataru shows he truly loves Lum by just how much he misses her. Also, Lum bugged the doll so she was able to hear every word he said.]]
* Kitsune-ko hears a fairy tale of a fox who defeats an Oni to rescue a girl trapped inside of a picture. When Kitsune next visits Tomobiki, one of the first things he finds is a photo of Shinobu. Having never seen a photo before, Kitsune believes she's trapped in the picture and his reaction is just heartbreaking.
* One chapter/episode has Lum's horns growing larger after [[BizarreAlienBiology a cow bite]], leading her to believe she's turning into a cow and avoiding Ataru as a result. Once he finds out, he breaks down and promises to keep her around even if she is a cow. We later see him building a cow shed in the backyard, still crying in grief. [[spoiler:Of course, it turns out she's not turning into a cow at all; it's just a side effect of how Oni antibodies react to uncommon viruses.]]
* In hindsight, the way Ataru is treated by everyone. At the beginning of the series, he isn't ''that'' bad of a lech (he did grope Shinobu's ass, but they were talking marriage literally fifteen seconds earlier). He even '''''saved the world from an alien invasion'''''… but a single misunderstanding on ''Lum's part'' caused his fiancée to go back on her promise, and everyone else to call him an idiot and a parasite.