She is voiced by Sanae Kobayashi in the Japanese version of the anime and Luci Christian in the English version.
Hitomi is a somewhat tall young woman in her early twenties with pale skin, a flawless complexion, long black hair and blue eyes.
Hitomi is a very aloof and stoic girl, and has been ever since she was a teenager. When she and Satou were in high school, she was rather shy and so the two would often hang out in the literature club playing cards. She had a fascination with dark conspiracies, and is partially responsible for Satou's extreme paranoia, and by extension him becoming a hikkikomori.
Her ravings about conspiracies can sometimes come off as odd, as she is shown to have a successful job as a civil servant. However, in the middle of the series, it is shown she is a bit of an outcast at work and unhappy in her relationship, being rather detached from her boyfriend Jougasaki, which is why she has no problem hanging out with Satou. She is also rather manipulative, as shown when she tricks Satou into taking part in a suicide pact, which he easily buys into as he regrets never going all the way with her in high school.
Hitomi foils Misaki in several ways. While Hitomi is distant and cold, Misaki is kind and persistent. Both are broken birds, but while Misaki hides her pain with a cheerful exterior, Hitomi doesn't even try to hide hers. Both have an equally toxic relationship with Satou, but for opposing reasons; Misaki sees Satou as someone pathetic enough that if she can become his reason for living she can make herself feel better. Hitomi meanwhile, from back when they were in high school, continually dragged Satou down to her level, playing into his paranoia and social anxiety.
While Satou has a habit of demonising Hitomi due to her weaving the idea of conspiracies into his head, he ultimately grows to realise that she is a person with problems just like him, and that he is just as responsible for his paranoia and nonsensical beliefs. Ultimately, when she attempts to commit suicide, as she believes that nobody needs or even wants her, she is talked out of it when Jougasaki arrives and proposes to her. This delights her, as she had been wanting him to say that for a long time, and she even saves Satou after he nearly falls off the cliff.