After seeing the full ending, serveral people have suggested that Kachi/Achi was only pretending to have lost her memory.
Although it could be said that it's not just an alternative interpretation, since her tone when she reveals her identity suggests that she was faking all along.
But that's something along the lines of Dubtext as the tone change isn't present in the original Japanese recording.
The backstory as explained in the European website further suggests that her amnesia was genuine. Her realizing that she has no memories is given in third-person omniscient narrator form, and happens before she meets Isa.
With Kachi's reveal, it's possible that the Nebulox are actually the good guys. Even though they are brutal, they were genuinely trying to protect their home from an extremely dangerous enemy who caused countless deaths and much damage.
However, it could be that Achi/Kachi had the right idea in the first place. She could have been trying to free humanity from the Creators, because they were brutal. Every time humanity tried to use diplomacy, they were wiped out. The Creators just wanted war.
Awesome Art : Yasushi Suzuki's official art for this game is rather breath-taking.
Not to mention another moment in which you kill a massive boss only to see it spit a newborn baby version of itself out of its mouth. This newborn instantaneously seizes control of a crane, grabs your partner, and then attempts to submerge them in lava. You free your partner, who seems to be emotionally just fine afterwards despite the constant danger of lava submersion, and then dispose of the little twerp. Why did it do this? You killed its mom, that's why! This moment is never brought up again afterwards.
The very final part of the ending battle by protecting Kachi's soul. Also, the music for the first part of the final boss battle.
"Hakai", the ending theme of Isa & Kachi Mode.
The system menu music.
Even Better Sequel: The first game is already great, if not requiring dexterity due to having to use an analog stick to control the cursor. This game is bigger (about 3 hours bigger), badder, runs at a slick 60 frames per second throughout the game (even in cutscenes!), and the pointer-based controls give you the cursor control of a Light Gun Game.
The character models and English voice-acting are good sources of this.
The scene in Stage 2 where Kachi asks Isa what "human" means. Dear god...
(Isa confirms for Kachi that humans have a head, two arms, two legs, etc) Kachi: "Oh, and a tail." Isa: "Sure." * awkward silence* Isa: "We don't have tails!" (Isa and Kachi laugh)
Stage 4, just about anything Ariana Shami says, especially "My Blood...is on FAH-YUUUUUUUUUUUUR!"
Narm Charm: There are those who like the blood on fire line.
Nightmare Fuel: The battle with the Infant Keeper in Stage 6. Your partner is being lowered by a crane into slowly-rising lava and you have to raise a pair of platforms to keep up with the lava while occasionally stunning the Infant Keeper to force him to knock your partner back up. If you fail to keep up, your partner is SHOWN BEING DUMPED INTO THE LAVA, resulting in a Game Over.
Scrappy Mechanic: Don't bother playing for score unless you're willing to draw out boss battles. Mind you, boss timers are often at least three minutes longer than needed to defeat whatever boss you're up against.
Kachi's Autolock ability. While handy against boss weakpoints, it has a habit of getting "stuck" on enemies you didn't mean to lock onto. This is especially problematic during the finale, where it can lock onto Deko while you're trying to shoot down his projectiles, doubly so since your attacks are completely useless against him, anyway.
Hard to pick just one, but the final boss on Hard mode is a fair contender.
And the final phase... essentially it's a harder version of the first game's final battle.
The Swamp Keeper, who's the final mid-boss of Stage 4 (or the second to last mini boss if you're playing the Japanese version) which constantly fires leeches at you. The endboss of that stage is actually easier.
The rescue mission involving the Infant Keeper. Doesn't help that it's also Nightmare Fuel.