Rushu is one of the first ten Demons who were born within the Krosmoz, around the same time the first ten gods were brought into existence. Having quickly eliminated his peers, he became the undisputed ruler of all the Demons who followed and still holds the title in the current day of the Wakfu Era.
As Rushu is more often mentioned in relation to his servants than shown in the media, his personality isn't entirely defined. His thirst for power and destruction is unquestionable, and his natural propensity for evil in general is no surprise. His involvement in season 2 of the animated series does not depict him as being particularly clever, although he is constantly plotting to take over the World of Twelve.
In the early days of his existence, Rushu saw the other demons as a threat to his own power and quickly began hunting them down. None escaped, although Mongrelamus did try to hide on some remote planet - as luck would have it, one that would later become the World of Twelve. Being the only remaining demon of the first generation and therefore the most powerful one, Rushu established himself as the ruler of all demons.
Even though the gods had decided that they wanted nothing to do with the demons and had taken the plane of Ingloriom for themselves, leaving the Shustuft Crust to Rushu and his ilk, the Material plane had remained neutral ground, and Rushu spent much time and effort convincing the fresh souls arriving in Incarnam to follow him to the Shustuft Crust instead of entering the Material plane. Well aware of his penchant for destruction, when the ten gods rediscovered the lonely planet and established life on it, they had Rushu sign a pact that would prevent him from ever leaving the Shustuft Crust. Though his agreement may come as a surprise, the fact remains that for a demon, a pact is only made to be broken.
Although he must remain in his dimension, Rushu convinced the gods to allow a certain demonic presence in the World of Twelve in the interest of fairness. When Xelor created his clock and named a guardian for each month, Rushu named his faithful Djaul as protector of Descendre. Through him, he continued to vie for more souls to recruit as Shushus and to cause mayhem in the World of Twelve.
Of particular note is the year 24, when Rushu's converts Karibd and Silar open a portal to the World of Twelve, allowing an invasion led by the master of the Shushus himself. Unfortunately for him, the attempt is thwarted, due in great part to Goultard's intervention (under his alternate personality of Dark Vlad). Shortly after, Djaul creates the city of Brakmar, a city of evil constructed in a single night and a bastion of malevolence to attract all the corrupt souls in the world who might serve the lord of hell. Bonta, the City of Light, is built in response and the two are destined to collide on many occasions (see Crimson Dawn).
In 505, the pact is broken once again, this time by the demon Esra'Ruoy'Dnim. Like others before him, he aims to find Dofus to bring back to his master, and his method of choice is to enslave the Amaknian populace and force them to search for the Dragon eggs, while he indulges in a life of laziness and constant feasts. That will ultimately be his downfall, as Allister's Ruse relies specifically on his boundless appetite. Once again, Rushu is left without a Dofus, and Esra'Ruoy'Dnim is kept in a prison and bound by the Heads for no Tails, unable to serve his plans.
The year 982 will be Rushu's last incursion into the Material plane to date, thanks to Qilby's underhanded assistance (see Wakfu (animated series) season 2). Once again leading his army through a portal, the lord of the Shustuft Crust finds himself confronted by Goultard for the second time. This time, the son of Iop is in full control of himself, but even more unexpected is the fact that he has taken over the role of Iop god. Despite his considerable power, Rushu is no match for the god of war and calls all of his Shushus to him, absorbing them to increase his own power and even the odds. While the outcome of their fight is uncertain, Goultard manages to send Rushu back through the portal right before it shuts, thereby repelling the entire invasion as practically all Shushus were inside the demon lord's body. In the final credits for season 2, the new Iop and Rushu are shown fighting for what is assumed to be a fairly long time.