In the light novel Volume 4, 『 』, with Izuna and Jibril found Plum, believed to be a Dhampir girl at the time, on the verge of dying of starvation. He asked them to give him their Soul Essence to avoid perishing. Jibril informed them that 'many bodily fluids contain[ed] varying amounts of soul essence'. Shiro helped him by telling him to lick Shiro's foot in order to consume her sweat. Plum stated that he likes the taste of Shiro's sweat. Jibril said it was because Shiro had a tough, strong, extremely rare, unique and noble soul that any common person who searched the entire world for such a presence would still never meet. After being saved by Shiro, Plum asked them to help his race.
Plum wears gothic clothing and a small top hat. He has blue hair and a black set of wings. He has also gotten a devilish looking tail.
Although initially believed to be a girl due to how he dresses, he eventually reveals that he is a boy and the last male Dhampir. He dresses like a girl to hide his actual gender.
He seems to be timid and easily prone to crying, but he is easily motivated at the prospect of obtaining fluids from Sora and Shiro due to their different attributions as they came from another world.
Like other Dhampires, he's very good at altering the perception of others with magic. He's also capable of creating a love potion so strong that it could pass as real love, something that neither Elves nor Flugels have been capable of despite their magical prowess over the other life form races of the Exceed. He is also good at shapeshifting, as he transformed into a scarf that gave Sora and Shiro the capability to fly during their game against the Flügel. This transformation has its own merits, as mostly Elves are the only ones who can use more than two spells at the same time.
He was able to fool a Flugel and a Phantasma using his Deception Spell after consuming a very little amount of blood from Sora. He even brags that at his maximum capacities, he could even fool an Old Deus (although he could have been exaggerating).
He proved himself a very capable strategist, having fooled even Sora and Shiro in his true gender for a while. He was almost able to manipulate the events of Volume 4 & 5 into a win-win situation where if Sora and Shiro beat Laira's game, the Dhampirs thereby are able to feed on Imanity who now shoulder the responsibilities of the Seirens. If Sora and Shiro lost, then Plum would simply state that it was an offering of food for the Seirens and could possibly allow the Dhampirs to feed as well.
Like all Dhampirs, Plum will die when exposed to the sun's rays. He can combat this by utilizing magic to block out the sun's rays. However, it takes a large amount of energy to maintain and thus is better for him to use it while in a dark area (a wooden box for the case of going to the beach with the main characters in volume 4) to lessen the amount of energy required to maintain the spell.
Ever since tasting Shiro's sweat, Plum has had a taste for the siblings' soul essence to the point of it being somewhat of an addiction. As such, the siblings often used it as a means of bargaining with Plum to aid them on their journey.