Aris, a robot seemingly designed for combat, is found inactive in ruins. She has a halo and is able to otherwise function as a "Student." However, this is implicitly confirmed to be out of the ordinary for the Kivotos. Both Momoi and Yuuka directly state that they used to be young children that grew up:
-Momoi, in the context of teaching Aris to talk more like a normal student: "I think I learned to speak just by listening to other people and watching TV" which is descriptive to human language development.
-Yuuka says she "loved playing games as a kid."
(You could go full tinfoil and propose that those are false memories or something. Even the good 'ol "turns out the whole thing takes place in Purgatory" wouldn't be debunked as they'd simply have memories from the "real" world before they died.)
Funny screenshot tax:

(It's Momover.)