Mikoto Urabe (卜部美琴 Urabe Mikoto) is the titular "Mysterious Girlfriend" of this series. Very little is known about her, as Akira Tsubaki 's occasional visits to her apartments have never shown her parents to be around. She initially transfers to Tsubaki's school and gives off an unsocial air, refusing to eat lunch with her classmates in order to sleep. One day, she breaks out in hysterical laughter during class, falling to the ground. When later telling Tsubaki about it, she reveals to him that she heard a voice saying that he would be her very first intimate partner. Urabe is very knowledgeable about the bond of drool that she and Tsubaki share. She displays her feeling through her drool, as her face is generally hidden behind her bangs. She has been shown to see Tsubaki's dreams through his drool, as well as use her own drool to cause him to have certain dreams. As the series progresses, she makes friends with Ayuko Oka , who also happens to share a drool bond with Urabe. Although she generally keeps quiet about her private life, she has been shown to be very sincere to her boyfriend, Tsubaki.
By nature Urabe is a girl of few words and very little facial expression. To the outside observer she seems to have very few visible emotions, this however is but a front. In reality Urabe is very much a normal teenage girl if some what different. She is shown to get embarrassed at the idea of anyone seeing her naked or even seeing her panties if her pantie scissors are not in them. She is revealed to have been giddily happy when Tsubaki told her he thought she looked nice in her swim-suit when the emotion is translated through Tsubaki and even becomes embarrassed when Tsubaki reveals he knows that she was happy even running away in embarrassment. She has very defined rules about her personal space not allowing Tsubaki to even touch unless she so wishes even attacking him should he try, this is contradicted by her during chapter 21 allowing him to touch her breast when she found out he had been thinking about it. She has a rather possessive personality, getting really jealous when he looks at other girls, including girls in magazines (especially Imai Momoka who looks just like her) where she shreds them with her scissors. Though she always has a monotone expression, she has been seen reacting erratically if she accidentally injures Tsubaki, which suggests that her control over her emotions is starting to crack. Urabe is in love with Tsubaki but has yet to tell him so save for in a very round about fashion, she painted the fact onto her body and had him taste her drool. She has an immense love for cats often becoming distracted when they are around. When a cat ignored her once she became so upset that Tsubaki cried when he tasted her drool.
Among her many other quirks, Urabe is notable for her skill with scissors. The scissors, which she keeps tucked in her underwear, can be used to dextrously cut objects at high speed, like making a nearby hedge resemble Tsubaki. The scissors are used as threats, displays of anger, and as a defensive measure. This is but one of the factors that keeps Tsubaki from being more forward and aggressive with their relationship, although he seems to be generally fine with following Urabe pace.
Urabe is a girl of average height characterized by her unruly thick black hair which falls over half her face. Ironically, under all that hair she is very beautiful to the point that it causes quite a stir at her school when she has her hair done up for one day. Despite her age, she has a very developed figure. She is shown on multiple occasions to have quite large breasts and a physically fit body. Tsubaki has often commented on how nice her body looks to him. When she is not in her school uniform she is seen to dress very nicely. She does, however, always wear a dress as she keeps her scissors in her panties. During winter, when it is cold, she wears a garter belt so that she can still keep her scissors hidden.