[spoiler]Yuki is methodical, stoic, succinct, and taciturn. She avoids speaking at all unless she is prompted to do so by a question directed at her or because her duties require it. Having a love for books and knowledge, she is often seen reading a book. After the school festival, she also became fond of working with computer software. Mikuru often feels nervous or intimidated by Nagato due to Nagato's abilities and her silent demeanor.

Yuki is not a typical human, but a "humanoid interface" or artificial human created by the Integrated Data Entity. As data that lacked a physical manifestation, her humanoid body established a communication link between her masters and humanity. She was sent to investigate the cause of the explosion of data that occurred three years ago which far exceeded anything they had observed. She is capable of superhuman feats and manipulating data, which can have an effect upon the physical world itself, an ability that Kyon refers to as "bogus magic." As a result, Yuki can have amazing athletic abilities (if she chooses to), program and manipulate software with incredible speed, create a baseball bat that always strike home runs, arrange for a student (Asakura) to mysteriously transfer out of the high school, change the weather, predict the future and many other things.[/spoiler]

[spoiler]Haruhi demonstrates a large array of natural talents. She is very athletic, a natural at many different kinds of sports, and her grades are described as "better than others". She has also demonstrated musical talents for both the guitar and singing (and, judging from the opening credits, the piano as well). Despite her many inborn abilities, she is rash, headstrong and often lacks common sense or good judgment. Until the formation of the SOS Brigade, she lacked any strong commitment or long-term interest in any single extracurricular activity. In fact, she joined every activity club in her high school only to quit them out of boredom the very same day, in spite of students pleading for her to stay due to her talents. It is because of her boredom with these clubs that she creates the SOS Brigade.

Haruhi parallels many of the ideals of Friedrich Nietzsche. The methods that Haruhi utilizes when founding the SOS Brigade also references Nietzsche's "The Madman". Haruhi's controls others solely by the force of her aggressive and competitive personality, echoing Nietzsche's concept of “will to power.” Haruhi almost completely ignores commonly accepted social standards in favor of what she deems to be the best course of action just as the Übermensch, or "super human," would. Haruhi is also capable of setting premeditated traps in order to obtain what she wants. For example, Haruhi obtains the SOS Brigade's clubroom computer by fabricating compromising photos of the Computer Society's President and blackmailing him. Similarly, she forces the recruitment of the SOS Brigade's members and holds daily gatherings within a clubroom that officially belongs to the Literature Club. The Student Council simply opt to ignore Haruhi's activities rather then having to deal with her directly. Similarly, many of Haruhi's classmates are intimidated by Haruhi and also choose to avoid having anything to do with her.[/spoiler]

[spoiler]She is a very sensible, honest, and forthright individual, and her down-to-earth practicality has charmed both the Host Club's members and its customers. However, her lack of hesitation in expressing her opinions can sometimes lead her to say, without any intended malice, things which others find hurtful. Her stubborn independence can also sometimes get her into trouble, as it typically doesn't occur to her to ask others for help even when she is in over her head, and has trouble understanding why people like her father and Tamaki worry about her. She is often bemused and sometimes irritated by the antics of the Host Club, but in spite of that, she has developed increasingly strong friendships with her fellow club members. She seems to be a bit dense where it comes to interpreting the subtler signals from people around her, especially when it concerns romantic interest, but is perceptive enough to tell the twins apart.

Haruhi admits that she has a low threshold for gender awareness; she was not actually trying to pass as male, but didn't feel the need to correct the others' assumptions. Her lack of differentiation between male and female gender roles was probably influenced by her father, who was originally bisexual and, saying that he could never love another woman, took up cross-dressing after the death of his wife. Whatever the reason, she is a fairly androgynous person who does not care much about the clothing she wears and seems to have little interest in romance. She enjoys taking care of household chores, particularly cooking, and is not easily frightened by things like bugs or ghost stories; her only weakness is a paralyzing fear of thunderstorms.[/spoiler]

[spoiler]Inspector Re-l Mayer of the Citizen Intelligence Bureau is in charge of investigating a series of brutal murders apparently committed by AutoReivs infected with the Cogito virus. She is also the favored granddaughter of Donov Mayer, the Regent of Romdo. Re-l accompanies Vincent on his journey in order to learn more about the mysterious Proxies.[/spoiler]

[spoiler]Despite these strange traits, she has the outward appearance of a young human teenage girl with an unusually large appetite (used as a humorous effect several times), most likely a side effect of her inhuman metabolism. However, she does start having more flashbacks of her experiences fighting Chiroptera in the Vietnam War and in 1920's Russia. Her oldest memories are from her birthplace at Joel's mansion called the Zoo in 19th century France. She was "born" in 1833 from a cocoon like object taken from a mummy of a Chiropteran along with another cocoon containing her sister Diva. She also can spend decades hibernating in a cocoon, only awaking for a few years before sleeping again. Whenever her and Diva's blood mix, the blood crystallizes and disintegrates. She is a skilled athlete and her weapon of choice is a nihontō (Japanese sword or katana) with bohi or grooves specially designed to spread her blood through its entire blade, making it a deadly weapon both for Diva and any Chiroptera made from Diva's blood. The first blade in the series is broken by Amshel, but she acquires a new sword from David with a red crystal at the base of its blade. This crystal is actually a piece of her adoptive father George's blood which crystallized when she killed him with her own blood in order to spare him the fate of fully becoming a Chiropteran.

Saya's human personality is usually that of a cheerful and optimistic girl but her suppressed vampire side (evidenced whenever her eyes turn red in contrast to her twin sister Diva's blue eyes) is infinitely more cold, calculating, and with a warrior's edge that brooks no foolishness. However, as of late Saya has had very little to be cheerful or optimistic about as she learns more and more about herself and the enemies that stalk her. Her greatest fear is perhaps finding out that she is indeed the same or much worse than the very monsters she now hunts as hinted by the grisly yet splintered memories she has of her dark, shadowy past.[/spoiler]