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.
Anthropomorphic Personification: Hikari and Akari are anthropomorphic personifications of extinction and survival respectively... they just don't know it.
But for Me, It Was Tuesday: The Real Hikari isn't actually all that evil, she's just doing her job. If it means psychologically breaking down the one thing that could actually kill you, then so be it.
Declaration of Protection: Takeru to Hikari. The twist in this case is that Takeru is an Extended Definition Organism designed to prevent the Seventh Mass Extinction... that Hikari is here to cause.
Determinator: Takeru will protect Hikari. Even if it goes against his Extended Definition instincts
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 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).
Monster of the Week: The Extended Definitions, which are monstrous versions of creatures made extinct by Hikari, resembling 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
Name's the Same: There's happens to be another anime with a young pair of romantically-linked teens that are also named Takeru and Hikari.
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)
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.
Like Takeru's transformation is not a shout out to Devilman... Sheesh...
Shipper on Deck: Most of the supporting cast, but Those Two Girls, who ship both Takeru + Hikari and Takeru + Mari, most vocally.
Sibling Yin-Yang: Ryo is a soft-hearted Cloud Cuckoo Lander, his little sister Kimi has it together, and takes things like bringing a hungry homeless girl into their house seriously...
Super-Powered Evil Side: Hikari's lighter-haired red eyed, much more forward alternate personality tasked with causing mass extinctions.
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.
Unfortunately for them, they haven't noticed they're Those Two Guys.
Tomato in the Mirror: It turns out Hikari is a goddess of death come to exterminate all life on earth and Takeru is an Extended Definition born to kill her.