I don't play a ton of metroidvania games (mostly just the actual old Metroid games), so I'm in position to place this in any hierarchy in the genre. That said, I did enjoy the hell out of it. They did a great job of taking the characters from Yohane the Parhelion and putting them in the game in a way that actually (mostly) makes sense.

Story: There's not much of a story. An underwater labyrinth appeared and your friends got trapped in it. You save them then there's an explanation at the end for why it exists. You'll definitely appreciate the characters and what little story there is more if you've seen Love Live! Sunshine and Yohane the Parhelion, but the plot is so thin you won't be missing out on much going in blind.

Gameplay: It's basic metroidvania fare where you explore different areas and have to collect new abilities to progress in certain directions or collect other things. Plenty of backtracking and all that, but you can warp to any save room you've been to at any time. There's also a crafting system for weapons and armor/accessories.

One of the things I think they did especially well for this game was making it so the other 8 members of Aqours weren't just damsels you saved and forgot about. Instead, they join you and you summon them to use their unique abilities to progress. Their abilities are also generally tied to LL/Yohane lore. You'll need to dig a bit deep into LL Sunshine to know why Ruby summons an all-devouring piece of cotton candy, but I assure you it makes perfect sense.

Conclusion: I paid $30 for this the day it came out in English and it took just around 7 hours to finish it and get the platinum trophy on PS5. I think it was a good purchase because it was genuinely fun while it lasted and I reeeeeeeallly want to encourage them to keep making these kind of tie-in games that aren't mobile gacha games. I'd play even my most hated genres if you slapped a Love Live or Idolm@ster skin on it.