Full Disclaimer, this is written based off of the first book only. As of writing this guide there is three available for purchase in English with a fourth coming out later 2020. This is also the first review I've ever wrote.
Let's start with a quick TLDR. For me personally my priorities in just about any form of entertainment goes something like this. Full disclosure, that doesn't mean that any one of these things don't matter in comparison, this is just my personal taste.
Character Design > Power Design(When Applicable) > Plot > World Design
So with that insight, here's what I would give those "specific" categories out
Character Design: 8.5
Power Design: 9
World Design: 9
The World's Strongest Rearguard is a great Isekai Harem Light Novel that takes a fun approach to some tried and true methods. I highly recommend it if you are a fan of the Isekai genre, or designs that are reminiscent of rpg gaming. It has some adult theme and suggestions, and I think is a good read for teens and adults. Here's some example titles that you could compare it to.
Dan Machi, Log Horizon, Final Fantasy Tactics, or a bit of a stretch "The Gamer"
Now with that aside I'll go into a bit more detail for why I gave the score for each of these. If that's all you were interested in, thanks for reading. For the rest of you, buckle up, this'll be a long one. I will of course do my best to remain vague to avoid spoilers.
We're going to follow the same order as above, so let's get right into it with character design. As previously stated, I put a lot of weight into good characters. That's of course like most things up to preference on a person to person basis. "The World's Strongest Rearguard" does an exceptional job in this department. The author does a good job of immediately allowing you to familiarize your self with just about every character you encounter. Everything from their personalities to physical appearance is mentally painted well. While not every character is depicted in full art, I still finished the book feeling like I knew what everyone looked like. Now that's great and all, but are they likable?
I at the very least, felt like they were. Our MC "Arihito Atobe" see's a lot of growth in this first volume. Like some other LN Isekai's he starts out still shackled by some troubles of his previous life (albeit less severe then most others.) However you get to see him become more comfortable and grow into his leadership role. His various companions that join him throughout the first book are introduced in appropriate ways, and quickly fit right into the overall "feel" of the story. No true "villainous" type has been introduced quite yet, so I cannot comment on how they handle "evil" characters.
Now with that all said, some negatives and in-between. While MAL does not list it as such, this is definitely a Harem. Most of all our side characters so far are Females, and totally enamored with our MC. While this isn't inherit ably a bad thing, it can be a huge downside for some. Me personally, I don't like un-justified Harems. That being where the MC is totally unlikable, and there's no real reason for him to be adored so heavily outside of "reasons." With that being said, I think this LN handles it quite well. Our MC is a likable guy, good looking, and has a in-universe reason for why people are attracted to him. (It's worth noting that this does not only apply to him either.)
My other complaint that ultimately stops this from being a 9-9.5 character design rating, is as follows. So most of the other characters in this world are all re-incarnated from a modern non fantasy day. With that being said, and in-case that's too vague. That means one way or another, they died, and were apparently immediately transported to this other world. All memories in-tact and looking no different from their original selves. Everyone seems to handle this quite well. While I can see the idea of all of those things together making it feel like you perhaps haven't truly died. A bit more shock and struggle with the concept would've been nice to see.
Ultimately I think that the Author did a great job, and I'm really looking forward to meeting more of these characters, and discovering what villains are like in this universe.
Hopefully not as extensive as the first Category, let me try and break this down. Basically when you're reincarnated in this world the first thing you are told to do is go and get assigned a Job, or in other terms a "class" I.E Warrior, Thief, Mage, etc. You can acquaint this with a system similar to Final Fantasy Tactics, or other similar Tactic Rpgs. You gain levels, and as you do are awarded skill points, to which you can allocate to a number of different skills that are available depending on you level. Now each of these jobs fall under three different categories: Vanguard, Mid-guard, Rearguard. Now as these names suggest these apply to the positions that said job would normally take in a "party". Again a term that for many video game players should be common place, but just in case, it's more or less a group of people that set out to accomplish a goal, in this case, exploring Labyrinths.
To touch a up a bit more on this concept. Every re-incarnated person is given something called a License. Which basically works as a menu screen from RPG games. You can view your status, and also form and manage parties here. The party leader quite literally sets people in one of their "Guard" positions here. So as you can see here it can get a bit MMOish. As you can tell, with a nine I found this system very fun, every new skill was well explained, and I got as excited as the characters when new ones became available.
Not going to go in too much depth here as, MAL rules tell me not to, and well there's not much of a need. The reason why this gets a lower score is because the over-arching plot isn't really explored too much in the first book, and what is, while interesting, didn't leave me theory crafting and craving more. I was just excited to go on more adventures with MC and the crew. I think it'll be interesting, but for me it wasn't terribly unique. I give it a 8 on MAL because decimals aren't allowed and I think it's more deserving of an 8 than a 7, plus some other factors that aren't really categorized by MAL helps it a long.
Again not to go to deep, but the world is set up pretty nice. Pretty much you have three different races so far. The Vaguely Mentioned Natives of the "Isekai World", Re-Incarnated Humans(Or Seekers), and Demi-Humans. I won't spoil much for what Demi-Humans are like, but they're pretty unique. In this world, there are things called Labyrinths which you can relate to dungeons from other games and series. In these dungeons are all sorts of creatures with unique abilities and designs. Again, the author has done a great job of explaining all of these things, and making every monster clear, and unique. Seeker's purpose are to explore these Labyrinths for the benefit of this Fantasy Country. They are awarded with Prestige, and Riches for achieving said feats. Not much else to say, I just think this was done extremely well, and I'm excited to see more.
In Conclusion / Final Thoughts
So once again, I think "The World's Strongest Rearguard" is a great read. It's enjoyable and tells a fun story. You can tell that there will be some terrible secrets lying in wait, but for now, we're just getting to know each other and that's alright. If you're a fan of the genre I think this is a must read. While there's not a ton of art in this first book, the stuff that's there is great. I think this would make a fun anime one day.
That's about all I've got. Thanks so much for reading if you made it all the way through. I saw that this series didn't have any reviews yet and was lingering just above a seven and I thought that was a shame. Hopefully this helps you decide if you want to purchase this or not.