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 elaborated prank (he got better, of course)]], makes it the more heartbreaking: if we don't take the [[ResetButton reset button]] trope into the serious consideration it deserves in this series, [[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 most popular episode includes one where Ataru breaks down in tears in the middle of an alley, clutching a small doll that Lum left behind when she suddenly left town. [[spoiler:While she simply had to renew her passport to continue living on earth, it's the first instance where Ataru demonstrates he truly loves Lum by how much he misses her. Also, Lum bugged the doll so she was able to hear every word he said.]]
* Kitsune 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 of the manga chapters has Lum's horns growing larger thinking she's turning into a cow [[BizarreAlienBiology from a cow bite]] and avoids Ataru for a while. When Ataru finds out, he broke down in tears and promised to keep her around even if she turned into a cow and we later see him constructing a cow shed, still crying in grief. [[spoiler: Of course, it turns out it's just a reaction from Lum's antibodies, she's not actually going to turn into a cow.]]