WARNING: The following contains spoilers for The Irregular at Magic High School Season 2, Episode 8, "Visitor Arc VIII," streaming now on Funimation.
The mysterious and widely feared Yotsuba Clan, the Ten Master Clans family to whom Tatsuya and Miyuki secretly belong, don't often flex their influence onscreen -- but when they do, they make it count. So it goes for the USNA as Colonel Virginia Balance, Angie Sirius's superior, who is forced into a difficult decision to ensure her own country's safety.
The Japanese media reports that its military "secured" a drifting USNA Navy vessel, claiming it was having engine trouble. In reality, the Yotsuba Clan has made a symbolic move to force the top USNA agents to negotiate. Tatsuya informed his Aunt Maya about the USNA military's troublesome pursuit of him, but it appears the Yotsuba already had the pieces in place to make a move of their own. Miyuki pretends to be oblivious around Tatsuya, afraid of him leaving her side, but she knows the full truth and will have to come clean to him eventually.
With USNA Colonel Virginia Balance already reeling from a series of setbacks in both of her primary missions, she receives a visit from a Yotsuba representative: Tatsuya and Miyuki's deceptively cute cousin Ayako Kuroba. In her own coded language, Ayako confirms the Yotsuba was behind the Navy vessel's troubles, then gives Balance a chilling proposition: remove the USNA military forces from Japan's shores or there will be direct conflict. Colonel Balance, firmly cornered, agrees to withdraw her forces in return for a potential future favor from the Yotsuba.
Amidst all the strategic maneuvering, Tatsuya visits the Pixie parasite on his own and learns the reason they infiltrate human and magician bodies: to breed. Each parasite cuts off a small part of itself which goes in and spreads throughout a body's blood vessels, exchanging itself for the human's blood. If the parasite successfully accesses the body's mental state, the human becomes a Vampire. If the process fails, as is more often the case, the human dies and the parasite must seek a different host.
This is crucial knowledge for Tatsuya. In previous battles, he and his comrades could only deal damage to the parasites' host bodies, or the Vampires. The parasites themselves have always escaped unscathed, but now Tatsuya knows exactly where they reside and therefore where to attack the host body: directly at the heart. The "why," however, still remains a mystery. Do the parasites breed out of a natural self-preservation instinct, or is there a more ominous goal?
Surprisingly, the Pixie parasite expresses regret. When informing Tatsuya of how the parasites breed, the parasite calls it "mistaken propagation" and says there was no intent to kill anyone. This expression of individuality happens again later, when Tatsuya, Miyuki, Honoka and Pixie confront three more parasites and attempt to talk things out. The parasites have come to take their comrade back, claiming in return they will no longer harm magicians. The Pixie parasite, however, does not want to go back, making the explicit choice to stay with Tatsuya. Whether any legitimate free will is being expressed is debatable -- the parasite remains influenced by Honoka's thought processes -- but it proves they are not simply evil beings.
Inevitably, a fight breaks out. Tatsuya uses his Trident CAD to launch a magic spell directly through the lead Vampire's body and into its heart. Blood spurts every which way from the host body, seemingly damaging the parasite as well. Meanwhile, Pixie defends Honoka from an attacker and gets a surprise helping hand. Honoka's hair ornaments begin to flash brightly, and a ball of electric energy appears to ward off the attacking Vampire. Most likely Tatsuya infused them with magic on purpose, thinking ahead of the curve yet again. This, along with Tatsuya insisting on the safety of all humans -- rather than just magicians -- is further proof that he is not the cold emotionless person he often believes himself to be.