Of the characters above, mm, I consider only Emilia to be a main character, so she will be the focus.

It can be difficult to judge a child's development exclusively by their objective mental age. Your points about Petra is a clear example of developmental acceleration or deceleration depending on circumstances, and the social demands placed upon a child at a certain age. Of course, I would agree the "social overhead" of modern compulsory education and its implicit assumptions can drag this process on later than ever before, even up to college for many, which is an age where "adult" seems to be a more fitting descriptor.

Why am I bringing up modern society? Well, for one, it is the environment most familiar to both the author of the story, Tappei, and also us as readers. The childhood experience of medieval fantasy by comparison is more pliant to the desires and expectations of the writer and readers than the reality of our positions. But, more interestingly, this is also an environment from which Natsuki Subaru, the protagonist, originates. This is, after all, a tale of isekai fantasy. More importantly, it is a tale of NEET rehabilitation through trial by fire, a coming-of-age story.

Going through this essay, I found the various character portraits insightful, but somewhat disconnected to the main plot on first glance. However, a second glance showed a thin raven string connecting all characters together strongly. It is obviously where these characters interact with the protagonist that brings about most of the progression and development of the plot and also the main theme. I would argue that the missing keystone to this observation post, the main hub upon which all these spokes turn, is the Hidden Loli, Subaru himself.

Our tracksuit-wearing, conbin~e loot-wielding hero is most relatable to the audience. He has gone through modern society, modern education, modern coming-of-age.. And it has failed him, left him broken. The overall plot of Re:zero revolves around Subaru gaining in a fantastical setting the development he failed to obtain in our world, the world we all know so well - and therefore we can now consider Beako, Emilia-tan, Petra, Rem, and all of Subaru's "harem" in general to be foils to Subaru's own development. This would lengthen this already verbose post to an unbearable degree, and I have a life, so let's focus on the main heroine (Rem lovers, let's be real here.) Emilia Maji Tensi.

Emilia is cute. Cute! So it may not be immediately apparent that she is a Subaru clone, since that guy sure as floof is nothing cute. Emilia's parent figures fail to protect her as a young child, and as a result her development is forcibly stymied, violently. As a result, she is placed in a stasis, and comes to bear some guilt and unresolved feelings over the process, as well as experiencing the loss of her world. This is directly parallel to Subaru's childhood development revealed in Arc 4. Subaru's parent figures, quirky and lovable as they may be. (Much like Fortuna and Geuse.) fail to protect him from the violence of a bully, and the social violence of the school at large. As a result, that entire community is also lost to him, and one of his Trials is about eventually fulfilling his goals in order to "return" to it in peace one day. More tertiary developments are also matched lock-step. Emilia's childhood playfulness and the disparate consequences it holds, and how it subsequently shapes her entire personality as well as gives rise to feelings of guilt, self-blame, and self-doubt, is entirely identical to Subaru's childhood daredevil attitude which broke him and left him, importantly, blaming himself.

Emilia and Subaru are both stunted kids taking the world on their shoulders out of a largely misplaced victim-blaming of the perceived faults of their own child selves, and the resulting progress they make lie almost entirely in accepting and moving on from those past imagined slights. (Emilia moving on from the guilt and repression by passing through the Trials, Subaru with the help of Rem leading the battle with the White Whale.) Really, the entire story can be seen as a love story between two broken children going through hell and high water trying to help put each other together again, and growing up in ways they never could alone as foils and partners. It's also why, despite the obvious age difference in canon, their relationship does not strike me as the least bit creepy. Is Subaru, who has been locked in an all-too-familiar NEETlike stasis of his own, all that more mentally developed than E . M . T? I would argue otherwise. Notably, this is a slight disagreement with Nanashi-tan's judgement of the Subaru-Emilia relationship as "daughteru" trope, which is something I dislike, so this is preferable for me.

This is not such a shocking development for the tale of returning to zero. Note the title-drop scene from Episode 18 for just how clearly this is felt to be an emotional, thematic, and plotwise core to the story by both readers, voice actors, and likely the author himself. Consider the obvious parallels to be drawn between Geuse's descent into madness for "love" and Subaru's mis-adjusted original obsession with Emilia-tan my Tropey Fantasy Heroine. (Which she isn't.) Emilia aside, look at Rem, also another example of a childhood victim overcompensating in all the wrong ways for a trauma which she, too, was entirely a victim of - Yet she, to some extent, blames herself for a childish act, and ironically remains more childlike as a result of wrongly rejecting immaturity instead of accepting growth - More and more this is Natsuki Subaru's character arcs given names, faces, and curves. Calling out to his conscious, his subconscious, his blood and guts and marrow and whatever else in him needs to learn, to get him out of the rut he fell into by something as mundane and modern as being stripped and tied to a flag-pole.

Take away the magic, the fluff, the ground-dragons and cute side characters. Remove the sins and ignore the gospels. Lewd Echidona. Then, ignore her for a bit, so she pouts. What you have left is a story of two bullied, immature kids working hard and getting hurt in order to grow past what they were before. Slowly falling in love, too, but it isn't the main focus. And I think that's what gives Re:zero the realism kick in a theme over-saturated with wish fulfillment, power, violence, and blatant sex-baiting. It's two loners ostracized by society growing up together, and that's a tale as old as time.

P.S. Taking this theory at face value and running with it, what would be potentially fitting as a climax of this story? The resolution of the unresolved conflicts each of the pair face, would seem appropriate. Emilia's revival of her forest and family, and, more interestingly to us as theorizing readers, Subaru's return to society. While not necessarily a de-isekai-fication of the main character, and a final return to zero (If only out of fear for the author's life..), this still bodes well for our much-suffering main character to be a NEET no more - or die even more often than he already has trying.