It's a cute, random curiosity. Primary school student Null has lost her sister Peta, so she builds a robot replacement of her. RoboPeta is big, white and fluffy, but also has spikes, drills, intercontinental jet boosters and spacegoing ability. She's also got Peta's personality, which means she's a gentle, warm big sister... who cooks sinister purple dinners and keeps pestering Null to attend school.
As an occasional one-off, it's fine, but don't marathon it. The show won't survive. It's mostly a bunch of surreal gags and wacky SF conceits that mostly exist to let the animators go apeshit. Well, that's okay. It's fun to look at, anyway. Null has amazing, impossible machines in her house. The first thing we see in ep.3 is a time bomb. (It's hard to drag Null out of bed in the mornings.) Ep.4 has a space rocket hidden in Null's school rucksack. This is silly, but entertaining. It's surprising when the show turns out to have multi-episode arcs, e.g. the space episodes or the goofy alien invasion. You might expect every episode to be a complete reboot, but no.
There's some character depth, though, with Null's determination never to go to school and of course one sister having been flattened by a truck.
The last three episodes, though, turn the comedy upside-down and explain it. There's a genuine surprise. The emotional depth is built on. It's demonstrated repeatedly that a certain character can't kill herself, although she's aware of that when we see her attempting it. Ep.12 even has the dreaded school. There's still silliness, but the show turns out to be much more real-world than we'd expected. That was interesting... but it doesn't change the fact that until then, the show had mostly been for laughs.
There are people who love this show. It hooked them. They got emotionally invested in the sisters, Null and Peta, while still enjoying the over-the-top SF antics. I can see how that could happen. Personally, though, I think you'd have to be fairly lucky to have that reaction. I certainly don't think the show stands marathoning, but you could try the odd episode and see if it grabbed you. They're only five minutes long, after all.