Spoilers, including from Light Novel (I didn't read it, we'll get to that).

I watched this series purely because it appeared randomly in a YouTube video of a person who doesn't even watch anime to begin with. I started the show not knowing it's a mystery, I thought it'd be a group of 6 defeating the Demon God (which doesn't happen in anime BTW). Don't get me wrong, Scooby-Doo is one of my childhood cartoons, I enjoy Detective Conan and I love Ace Attorney games, so I can appreciate a good mystery. It wasn't, but I had fun watching it because of how ridiculous it was.

Let's start with visuals.
This show features a significant amount of CGI for enemies and projectiles. It felt jarring to me.
The 3D usage of camera was questionable. In first episode, Nashetania talks with Adlet in prison. Normally, you would see a bunch of cuts whenever each character speaks, but for a part of this scene, the camera was smoothly moving between the two for no apparent reason. Okay, it isn't important, I admit, there's worse. When all the 7 characters are in the temple, there's this bit when all of them are in the circle. The camera kept rotating in the middle of that circle as if it wanted me to feel nauseous. We'll get to nausea later.

Music exists, it wasn't annoying, I didn't pay much attention to it, I won't remember OPs and EDs.

With that out of the way, characters sorted by the amount of favourites, along with some plot:

• Fremy is this half-fiend that was hurt by fiends and wants revenge. She wanted to kill all the candidates for Braves so that she can deal with the Demon God on her own. That's also why she doesn't trust anyone and it makes sense. I'm not surprised she's the most popular as she got the most attention, she doesn't end up the 7th Brave and she has some cute moments once she starts trusting Adlet.
• Adlet (I'm sorry, but I couldn't stop calling him Aglet) is the main character. He keeps saying he's the strongest man in the world and while I get why by the end of the show, it's still annoying. Besides this gimmick, he feels like an average positive protagonist. He falls in love with Fremy and it doesn't make any sense. I find Fremy cute, but as a character, not as a person I'd like to actually spend time with. Sure enough, when I googled what's the ending of the series (since Rolania appears), it turns out he was hypnotized to love her. In other words, this was meant to be a plothole solved in later seasons, except this anime aired 9 years ago and I don't expect it to ever have a sequel.
• I don't understand Hans' popularity. He's a messed up catboy and that's literally it, at least in the anime.
• I thought Nashetania would be the most popular character, but it isn't the case since she's the 7th hero and her personality suddenly switches. Her motive sounds dumb. Surely humans and fiends can live together after you sacrifice 500,000 people, I've never heard similar concepts in anime ending with human population decimated /s If she wasn't a villain, she'd be my favourite. Her interactions with Adlet before they arrived at the temple were cute.
• Chamo is 14 years old, but she feels like she's just started primary school. A Wikia about this show agrees with me and I really don't understand what was LN author's point. Compare this to Adlet, Nashetania and Fremy being just 4 years older, Goldof only 2 years older, to see how ridiculously this number is - at least give me some explanation why it's this high. Otherwise, I enjoyed her personality of being really fast to jump into conclusions, maybe she's even bloodlust. However, why the heck does she generate minions by puking? Was that really necessary?
• Mora Chester has the biggest chest, I wonder why ? She's a voice of reason and a MILF bait, even Chamo calls her a granny. As with Hans, there's really not much to say.
• Goldof's only trait is being Nashetania's knight, he protests against her once and that's it.
• Rolania appears at the end of the season so that we can have an open ending and a 2nd season, which would be, again, about finding who's the 7th Brave. I find it pretty lame that the author couldn't find any other way to create a mystery. She doesn't even have an AniList as of writing this.

I think listing these characters showed a significant problem: half of them are incredibly flat (well, except Mora, she has a chest). I don't expect to remember Mora, Hans or Goldof a few months from writing this.

Now, for the rest of the plot and pacing. Episode 1 introduced a lot of lore that is completely irrelevant to the story and could be introduced in 2nd season, if needed. I don't care it's the third time 6 Braves appeared if that doesn't matter in these 12 episodes. I appreciate that Adlet's actions had consequences and Nashetania actually had to take him out of the prison. 4 episodes pass, I'm introduced to Fremy, that is wanted dead by Nashetania and Goldof. They fight, but Adlet listens to Fremy's story and tells them to stop. Then I realized it's a mystery show and oh my, I didn't enjoy it.
On paper, it seems fine, Adlet has to figure out how it seemed to be a locked room. You really don't know who it might be until episode 11. I think the solution made sense and there were hints as to what happened (episode 12 even shows you the examples).
What I didn't enjoy was that the main suspect was, most of the time, Adlet. If it was actually him (and turns out, season 2's conclusion would be that he's 7th!), it'd be justifiable. However, the anime clearly shows he couldn't have activated the barrier, so all these fights against him were just to show each character's powers, it seems. I know that, if you removed the action scenes, the mystery would last just an episode, but I feel this time could be spent better.
Another issue: does it matter? You die 24h after you enter Demon God's realm, so it would be obvious who's the impostor.

Why did I enjoy the show? I think it's a matter of how everyone hated the protagonist for (from viewer's perspective) no reason and how nobody stopped them from attacking each other. I think my rating would be higher if Adlet wasn't proven innocent from the start. The anime's intended message is probably that you can be strong by showing you're right, despite what everyone else says, but I didn't buy it while watching this series. It's hard for me to recommend this, but I had fun nonetheless.