Affably Evil / Punch Clock Villain: Ionoious. He behaves exactly the same both before and after his mission to aid Hikari in her destruction of the world: polite, friendly, accomodating, even apologetic about the job he has to do.
Crazy Jealous Guy: Daijirou and Shinichi, Takeru's other guy friends, also don't like that Hikari seems to have appeared out of nowhere and is hanging out with him. Hilariously, in episode 2, after Takeru's aunt and uncle have a talk with him, those two guys also drag him out afterwards to have a talk with him as well.
Energy Being: Hikari and Akari's true forms, as seen when Akari transforms her arm into a Laser Blade to stop Evil Hikari from destroying the world.
Evolutionary Levels: Of a sort. Extended Definition is essentially instant evolution to one's maximum physical (and metaphysical) limits.
Fanservice: The series averts Barbie Doll Anatomy when it comes to breasts, and isn't afraid to flaunt it. The series even lampshades it: in one scene, Jennifer gets kinda cozy with Takeru, and a guy screaming "Aww yeah!!" can be heard in the background.
Festival Episode: Obon festival, to be more precise. Made plot relevant since it is during this festival which people honor the deceased that Hikari first comes to understand death.
Hard-Drinking Party Girl: Both Jennifer (who spends about 80% of her screentime drunk or drinking) and Ionious (who Jennifer is always forcing to drink).
Hot Scientist: Jennifer to a tee, being a busty blonde American woman in a lab coat.
Monster of the Week: The Extended Definitions, which are monstrous versions of creatures made extinct by Hikari. The ones encountered resemble an Anomalocaris, a prehistoric fish, a theropod dinosaur, and Takeru, a Homo sapians.
Mood Whiplash: The tone of what begins as a silly Fanservice anime turns very serious starting about episode 8.
Motifs: As well as the insects, DNA-type patterns show up quit a bit
Phenotype Stereotype: Jennifer, although to be fair, the Moe aesthetic of the show makes everybody look ambiguous in nationality - the Japanese cast for the most part has realistic hair colors, and the two divine-like girls Hikari and Akari have more improbable hair colors (bright orange and purple-red, respectively)
Robot Buddy: Both Ioneous and Kuon, the latter being a shard of the former.
Rule of Three: Those Two Guys are convinced that a third girl will fall from the sky, solely because of this. It doesn't happen, of course. Technically, there already are three girls: Hikari and Akari combined.
Super-Powered Evil Side: Hikari's lighter-haired red eyed, much more forward alternate personality, tasked with causing mass extinctions, is far more powerful than her appearance lets on.
Takeru's transformed (Extended Definition) form comes with more feral instincts, leaving him to brutalize the various monsters he encounters. It's also driven by instinct to kill Hikari at the end of the series.
Super Mode: When Takeru takes contol of his Extended Definition form.
The Talk: It's implied that Takeru got this from his Aunt and Uncle, as well as Those Two Guys. Of course, he did just bring home a girl who he found naked in the woods, and is blatantly infatuated with him.