Kou Kunugi takes the supporting part of his role as supporting character very seriously. Best friend to Kaito Ayase, he has a front row seat to his romcom protagonist antics and gives his all to propping up his romantic ventures and staying out of his limelight. His plans to stay in the background fall apart when one day on the way to school he saves a girl from an exhibitionist and that girl just so happens to be his best friend's sister. Mingling with the protagonist's little sister is a strict no-no, but he mingles with her anyway after committing to helping her overcome the mild case of agoraphobia she's developed, but the consequences for doing so dredge up painful memories he'd rather keep locked up and buried in the past.
Ever meet somebody who tries to get others to bust a guffaw but are so over-the-top, hammy, and random that rather than laughing you stare at them with furrowed brows and wish they would go tuck themselves in a box somewhere? That's what I did reading this book. Making humor the mainstay of a narration style is a fast and easy method for getting on my good side, but the desperation on Kou to get my approval is so thick that I could peel it off him like a scab. He's in the habit of punctuating witticisms—which are lacking the wit—by enclosing them in parentheses, and his focus is all over the place. In one scene he's reading a hair care magazine and prattles on and on about shampoo. Given the absurdity of the contrast, I want to have found this Shandyism comical, but it felt like I got a call from my mother who gave me a spiel on how much weight she lost after drinking vegetable smoothies when I was in the middle of getting my taxes done.
An even worse routine is his constant reminders of everybody's roles in the larger story. I love meta narratives and trope deconstructions more than the next 1,000 people you'll meet on the street combined, but he can't go three pages without bringing up that his best friend's the protagonist and is therefore ordained by the gods themselves to score all the pussy he could ever need. Assembling this romcom framework and then viewing the narrowed world through it I could get behind, but he can't hold it steady, so the frame floats so often into my field of vision that it's akin to a jackhammer being used right outside my bedroom window.
Ordinarily, I'd leave those criticisms there and move on to the next hit on my list, but an item I left out that I would've put up is how wishy-washy Kou is about his framing. When he's with his BFF, he's a mouthpiece for the great and mighty romcom protagonist, but then he has the gall halfway through the novel to say, “This isn't a novel.” I was asking which it was, are they or are they not in a work of fiction, but the last couple of pages address this glaring discrepancy, which threw me for a small loop with how (mildy) self-aware this series is. The reveal's not mind-blowing or executed with the greatest of grace, but it at least shut me up.
However, bringing up this twist brings up a new slew of criticisms I have, which means I'll have to spoil the whole story, so this is your spoiler warning, but you shouldn't worry about wanting to read it yourself, since I'm only rating this book a 5 out of 10.
To summarize, Kou was born in our world but then was inexplicably teleported to an edgy isekai world where best friends become monsters that have to be slain with his own two hands, and then he inexplicably teleported back to our world with no memory of having been born in our world because he cast an amnesia spell on himself. His framing of Kaito as a protagonist with his own harem is a coping mechanism. Kaito's just a normal boy who's not all that he's cracked up to be, and all that talk of harem heroes is an expectation foisted on him as a stopgap atonement for the sins Kou committed in the edgy isekai world. It's surprisingly thoughtful and paints Kou as more nuanced and intelligent than he lets on, though like I said and will continue to say with how shoddy the execution is, it doesn't turn the reader on their head so much as slightly tilt their head.
The problem with this self-constructed light novel romcom narrative is that he's already in a light novel romcom narrative, and I don't mean that because it's a physical light novel. How the characters act and react are all in tune with how I expect other light novel characters to act. The lunch line is indistinguishable from a mosh pit, a loli unironically says “Uwuu,” and girls fall in love with the boys simply because they were damsels saved from distress. It tries playing an Uno Reverse Card when one of the girls confesses that she didn't love Kaito the way she thought she did, but it's an illegal play since she already fell in love with him after he saved her from a bad dude.
It's beyond me why The Sidekick Never Gets the Girl, Let Alone the Protag's Sister! shied away from a realistic depiction of the world. The backstory I came up with is that it'd been locked up in a closet since birth and its only exposure to the outside world were secondhand My Youth Romantic Comedy Is Wrong, As I Expected volumes. But if it had the Book Girl series, it might've fared better. In Book Girl is a genki girl, which is right at home in a light novel, but her hyperactive, adorable persona is a facade to throw a quilt over her unsympathetic, unempathetic personality. Sorry it's not a 1:1 comparison, but it can still carry over. The genki girl is the romcom lens Kou views Kaito and his female company through, and the emotionally cold girl is normal human beings.
Since I brought up a psychopathic character, might as well use that for a segue to discuss Kou himself. Having lived and lost in an edgy isekai world, he's an edgy person himself. Why, he's comparable to Kiyotaka Ayanakoji when he hatches a plan to manipulate others to get Kaito's little sister back in school, despite the heartache it puts one such pawn through. What that plan is it's impossible to say because not only is Kou lacking compassion, he's lacking even the barest glimpse into what his plan is. He says he has a plan, then skips to the consequences of said plan. Does he not realize what an aggravating tease that is? Imagine if Re:Zero did that. Subaru gathers up a platoon to slay the White Whale, they arrive at the battlefield, prepare for a life-or-death struggle, and then everybody throws their hands in the air and shouts “Huzzah!” with the monster's corpse in the background. I imagine the reason The Sidekick Never Gets the Gal, Let Alone the Protag's Cousin! skipped the enactment of its plan is because it's not smart enough to come up with even a quarter-baked plan.
Jury's out on whether the humor in this book can be legally classified as humor, but it does something right by being there in the first place, almost. See, there's this deal with comedians, and that deal is that they're the headliner of a Saturday night venue, but look for them backstage and you'll find them draped over a crate because they forgot to take their antidepressants. Jokes are among man- and womankind's greatest inventions, and some men and women use laughs to mask personal pain and grievances. The contrast between how Kou acts and how he feels is all but spot-on for using comedy as a band-aid, but the reason I keep using words like almost and all but is because he's rolling with the mood more than he is being strategic with his masquerade. You could argue his humor is a complete mask, which I wouldn't disagree with, probably, but I don't get that vibe. He doesn't joke about his pain the way someone fearful of death might take jabs at Mr. Scythe himself, and he doesn't make cheeky remarks to redirect his focus whenever bad memories come bubbling up. What should be two sides of the same coin are sawed in half and separated by one glossy pane.
My last nitpick with Kou is his treatment of women. To summarize, there's a power dynamic where if the girl's being a bitch, he cowers in a chair, and if the girl's a decent human being, he's a total a-hole to her. I would grab him by his lapels and ask him what stick bug crawled up his rectum, but I already know the answer. The Sidekick Never Gets the Madam, Let Alone the Protag's Mother! was too afraid that Kou would be a generic, boring hero if he was all right to people, so it spiced up conversations by singing Eeny, meeny, miny, moe with those coin halves to decide who would be the dick. It is possible, believe it or not, to have an interesting conversation with someone, even a stranger, without resorting to picking on and demeaning them, which further proves this series's social ineptitude.
Learning how to talk to people would be one fix to this fault. Another would be if this book didn't introduce so many characters. Plenty of series I like have plenty of characters, but they stagger their debuts for when they're needed so that the audience isn't bombarded with side characters made up because the plot didn't know how to finish a scene. The common critic would complain how this thins out the limited development available to this unnecessarily large cast, but I took bigger, if pettier, issue with how everybody's names start with the same letter. Maybe it's because I wasn't raised in Japan or maybe it's because I'm an idiot, but the alliteration of foreign names caused my brain to freeze up as it had to fetch the identity matching whatever name was brought up. Kou and Kaito I could deal with, but also in their friend circle are two girls named Kiryu and Kotou. Thankfully, Kaito's little sister's name bucks this trend, but she has two classmates, brought up at the eleventh hour, whose names are Mikura and Murata. It must think parents two decades ago sorted themselves into groups and swore an oath to name their firstborn after their group's designated letter, all because My Youth RomCom got cute with naming its two heroines Yui Yuigahama and Yukino Yukinoshita.
Like any mediocre light novel, it comes with side stories that hardly add a thing to the overarching narrative. There's little more to say than that they're a pointless waste of time. The first side story is Kou telling us about when he dictated that Kaito would be the romcom protagonist he supports, which is repeating what we already know but in verbosity. The second side story is Kaito's little sister going “I'm Kaito's little sister!” ad nauseam. It does offer up one faintly intriguing morsel of a stray cat apparently leading her to meet Kou, but it's nothing more than a wind-up that won't be explained until a later volume I won't read because reading this first volume was like sliding sugar-coated railway spikes down my esophagus.
Someone ring up the police because several light novel series have met grisly fates and The Sidekick Never Gets the Mistress, Let Alone the Protag's Aunt! is to blame. It couldn't decide if it wanted to be a romcom or an edgy isekai, so it stitched both together, and what we're feasting our eyes on is a monstrosity wearing the skins of those murdered series. It's a friendly monstrosity that's up for a night of roasting marshmallows, but don't trust being alone around it for too long, unless you're prepared to push it to the ground, spit in its face, and let it know who's the boss of the outside world.