Kurumu is a Succubus attending the Yokai Academy, first she tried to use her powers to charm all the boys at school. But after she fell in love with Tsukune she gave up all that and now only uses her Physic and charm to try to make him hers.
Kurumu shows up early in the second episode, kneeling in a very vulnerable looking position next to a small pond. Tsukune, being the trusting/caring individual that he is, offers to bring her to the infirmiry. Kumuru then springs up, rather dashing any thoughts of her actually being hurt or sick. It is then that Tsukune becomes acutely aware of how large Kurumu's breasts are. The writers also seem to have a thing for it, as any simple movement causes Kumuru's chest to bounce.
Tsukune takes her to the infirmiry, but along the way Kumuru is practically throwing herself on him, eliciting some rather "observant" thoughts from Tsukune. Mostly along the lines of "so... so soft..." I'll let you guess what he's talking about. When the "pair" meets Moka coming along the same path, Kurumu sets herself in front of Tsukune and gets him to look her in the eyes. Tsukune is then hypnotized and led off on Kumuru's arm.
Kurumu states to Moka that it is her plan to make the entire school of boys her slaves, revealing in the process that she is a succubus (against school rules). Leaving with Tsukune and letting an angry Moka stew over the situation.
In the infirmiry, Kurumu attempts to have Tsukune kiss her, pretty much a death sentance for him if he does, as his entire being would be dedicated to satisfying her, Tsukune resists by hugging her and refusing to look at her in the eyes. It is now that Moka bursts in and startles Kumuru enough to get her off of Tsukune. As he is weak from the aftereffects of Kumuru's "charm" he falls off of the bed, grabbing Kumuru's panties on the way. Awkward pause... massive nosebleed from Tsukune.
Kumuru gets mad and attempts to slice both Moka and Tsukune apart out of revenge. Needless to say she fails after Tsukune removes Moka's rosario and vampire Moka drops her with one kick.
As vampire Moka moves in for the kill, Tsukune physically blocks her. Sparing Kumuru in spite of what she had attempted to do. Saying that it is only in her nature. Ironic how Tsukune seems to make practically every single girl indebted to him in some way. Even though he clearly does not.
The very next day Kumuru is waiting for Tsukune with a box of cookies(side note, Kurumu seems very good at baking sweets and other such things, although she usually blends in somthing like a love potion or somthing along those lines) , claiming that she has made him her "destined one" aka, the one she wants to spend the rest of her life with. As Tsukune is babbling his half-baked negatives Kurumu grabs him and buries his face in her chest. While arguing with Moka at the same time. This continues until Tsukune almost suffocates. As a matter of fact, Kumuru has a very pronounced habit of either holding Tsukune between her breasts, or draping them over his head.
Kumuru also carries the twin titles of "largest breasts for female lead" and my personal designation of "Miss Waterworks" as she is the one that cries the most through the entire anime. Three times in total (two if only counting times of genuine sadness). Once when vampire Moka is about to kill her (which Tsukune stops). Once when she is forced to burn the newspapers that she made with Tsukune, and the third time when she lerans that Tsukune was actually a human. That bit of knowledge still does not shake her love for long.
As typical of her monster type, Kumuru is not the least bit body shy. As illustrated by the many times she presses her chest against Tsukune. Perhaps a better example of this is one episode where Tsukune is framed as a peeping tom, when told that Tsukune was sneaking a look at her panties she shrugs and says "all he had to do was ask".
Somthing else interesting, as Kumuru's mother is also a succubus, Kurumu, on two occasions, must pry Tsukune from her mother's grasp. Once by actually physically assaulting her own mother.
Kumuru is the obejct of affection of a particular male monster worthy of mention. As stated before, any slight movement on Kurumu's part causes her breasts to bounce. The writers apparently decided to make a small joke out of this here. The boy that is interested in her happens to be a very low grade monster that is quite fat. The moment in particular is when the boy in question is following her through a hallway. The camera focuses on Kurumu's bouncing chest... then on the boy's bouncing gut. He proceeds to stalk her around the campus for the remainder of the day. Every time Kurumu sees him he is posing and holding a rose, obviously attempting to look attractive. Despite the "sparkle effect" he fails miserably by the fact that he was stalking Kurumu all over the entire campus. On a side note, Kurumu has issues with stalkers, frequently berating Mizore for stalking Tsukune. When the boy attacks Tsukune for holding Kurumu's affection it is Kurumu that beats the boy up (rather difficult to do considering that the boy's monster form is just a blob of jello like flesh covered in slime).
Kurumu has a particular rivalry with Yukari, frequently using the fact that the younger girl is flat-chested to infuriate the witch. When the entire cast takes a trip to snow mountain Kumuru and Yukari get into a rather heated argument over how their respective breasts affect their skiing (neither of them are any good at it). Kurumu's comment is that Yukari would be better off sliding on her flat chest than trying to ski (Yukari then proves the point by sliding around on her belly). Yukari's response is that Kumuru would fall flat on her face from the extra weight on her front. This is never proven and Kumuru really never seems to be bothered.
Oddly, considering her apparent popularity with the editors, Kurumu is the only girl that doesn't have a single episode focused mostly on her. Yukari has the one with the nurse, Mizore has her curry episode, Ruby has the multi-part one, and Moka is pretty much the subject of eveything else. Unlike the other girls, Kurumu actually talks directly to the camera, and I believe that this is called breaking the fourth wall, at times, one time of which she is speaking about how "this episode should be all about me". Against this prediction, it was not.
One last point of note, when Moka is about to drink some of Tsukune's blood (and they aren't alone) Kurumu is the one that stops them both with a loud (Kurumu is very loud on average) shout. She claims that it isn't good for Tsukune's health for him to be drained so much, but she probably just doesn't like seeing Tsukune and Moka in such a romantic and personal setting.