Anti-Climax Boss: Spiritual Larsa in Futari Black Label's God mode, believe it or not. While she is still of course the game's ultimate challenge and she gets a far more dramatic presenation than her Ultra mode counterpart (which simply reuses the standard boss BGM and doesn't involve her crossing the Bishoujo Line), she is overall easier and her final phase actually has safespots; she summons a circle of pods that release bullets and with some careful maneuvering, you can stay between two of the pods as they move and never get hit. This was not fixed in the 360 port.
Awesome Music: Composed by shmup soundtrack legend Manabu Namiki, as well as Masaharu Iwata (composer of soundtracks for Soulcalibur IV, Final Fantasy XII, among others) and Kimihiro Abe for the sequel. There's also the Double Arrange album which re-arranges the tracks from both games through a variety of other famous composers.
Speaking of the prior final stage theme, there's also On The Verge Of Madness, which packs some oddly upbeat Chinese instrumentals but may be enough for you to take on Queen Larsa to stop her once and for all. For any Armored Core fans (and the despite the lack of the prior Chinese instrumentals), Kota Hoshino did the arranged version.
Complacent Gaming Syndrome: Most players regard God Mode as the only thing worth playing in Futari Black Label.
Contested Sequel: Well, Contested Expansion Pack: Futari Black Label, due to the increased score penalty for bombs and other changes in scoring.
Even Better Sequel: — Mushihime-sama was interesting for its different premise and music, but otherwise had rather simple gameplay and visuals, especially compared to other Cave games. In some ways it was even simpler than DonPachi. Then Mushihime-sama Futari came along, turned the music up to eleven, made big improvements to the gameplay, and is considered to be one of the prettiest Cave games of all time.
The legal warning on the original game said that it was for use in Japan only and exporting to other countries would be illegal. The warning on the Steam version, on the other hand...
In the 360 port of Futari, you get an achievement for getting a Game Over while the special Stage 5 1-Upis on the screen.
Good Bad Bugs: The "counter banking" technique in the original game, allows you to get much higher scores than intended in Maniac and Ultra modes.
Low-Tier Letdown: Almost no one uses Reco-Abnormal, and for good reason—her shots are difficult to master, and her speed when using her focus shot is faster than her normal speed, in a defiance of usual Bullet Hell conventions.
Memetic Badass: Queen Larsa, due to her status as Futari's notorious Final Boss, especially on Ultra difficulty where she has a True Final Boss form and becomes known as "THE HARDEST VIDEO GAME BOSS EVER".
Memetic Mutation: "THE HARDEST VIDEO GAME BOSS EVER!"Explanation The title of a rather infamous Futari video showcasing the True Final Boss on Ultra difficulty, which drew a lot of attention from people unfamiliar with the Bullet Hell genre. Note that many shmup fans do not consider Spiritual Larsa to qualify for that title, possibly because the Black Label version wasn't showcased yet at the time.
Moral Event Horizon: In Futari, Queen Larsa goes batshit after her son dies in the previous game. Understandable by villain standards. She also decides to send her remaining child Palm out to die, dismissing him as someone she can replace anyway.
Most Wonderful Sound: As she is one of the most infamous bosses in Shoot 'Em Up history and she's by far the most morally broken character in the series, it's incredibly satisfying to hear Larsa's death scream when you finally defeat her.
Nightmare Fuel: Queen Larsa. Her older son Aki dies and she threatens to kill Reco, who she hold responsible for his death. What does she do to Palm, her younger son, when he tries to convince her that Reco did nothing wrong? Disown him and leave him to be killed by her forces. When he comes back to confront his mother, she attempts to kill him with the same amount of firepower she would use on Reco; in other words, she thinks so poorly of her son that to her, he's simply another bit of opposition who needs to die.
The Nintendo Switch port of Mushihimesama may well be the best version of the first game, featuring all of the content of the 360 and PC versions under one purchase and with only 3 frames of input lag when using a wireless controller (4 with a wired one).
The 360 port of Futari by M2, featuring a Novice mode, both the original revision of Version 1.5 and an Xbox 360 version with upgraded graphics, a tag-team Arrange Mode, and Black Label albeit as a DLC purchase, and all with a mere 2 frames of input lag. It is widely regarded as the best CAVE port on Xbox 360, as well as one of M2's finest works and at the level of their later M2 ShotTriggers releases.
Porting Disaster: The PS2 port on the other hand, handled by Cave itself, doesn't emulate the arcade version's slowdown properly, and the game is downscaled in screen resolution, causing it to look blurry when played with the game window oriented vertically to fill the screen. The Steam port, as of this writing, is also plagued by a number of irritating (but not game-breaking) bugs, which CAVE has assured players that they will soon patch.
Scrappy Mechanic: Futari's Black Label version imposes a massive penalty to your counter when bombing, more than the penalty for dying. This seems to be a recurring theme of games designed by Tsuneki Ikeda, who seems to make it his Creator Thumbprint to heavily penalize bombing.
Signature Scene: Much like with other CAVE games, the True Final Bosses — Aki in Mushihime-sama, Larsa in Futari, Queen Larsa in Futari Black Label — are this. Incidentally, Spiritual Larsa is better known than her Futari Ultra incarnation despite being easier due to her more grandiose presentation, her dramatic makeover, and the unique BGM she gets (In Futari Ultra, Larsa's final form BGM is simply the standard boss BGM).
The Final Boss fight of the original game. Neither of them really want to fight the other, but Aki is bound by his divine duties to fight her when she sees him.
In Futari, if you make it to Larsa as Palm, Palm tries to convince her that Reco's a good person and that Aki's death was not out of malice. Larsa just smugly tells him to die, and goes full power on him for their final showdown. Palm kills Larsa to defend himself and everyone else she's waged war on, and she never expresses a shred of remorse for her actions, only sorrow for Aki to her last breath if she's defeated as Spiritual Larsa.
The first boss of Futari has an annoying bit of RNG in its first phase, where it'll perform either an easy attack with a glaring safespot, or an incredibly annoying attack that requires you to weave in between fast-moving bullets with inconsistent slowdown. Since it's only the first stage, bombing the attack isn't really an option, so resets are very common if it uses the latter attack.
Futari's Ultra Mode's Larsa has an extremely tough final attack; two different, spiraling, extremely dense streams of purple bullets, with falling pink bullets entering the fray if the attack is survived long enough. Multiple bombs are practically mandatory to pass it, but Larsa is also bomb shielded, and the attack has RNG, where sometimes the two streams overlap immediately, forcing an immediate bomb.