Hildegard is a member of Legion of The Elite Species commonly known as Investiture Beings. He was an antagonist throughout the Blue Dragon Trials of The Seven Shadows series but later during the end choosed the right side.
Hildegard is comprised of two halves -- one being male and the other being female. In their base form, they appear as a normal human being, but they have two personalities and can switch being the two at will with the only difference being a masculine and feminine voice.
The male half seems to be largely serious while the female half is more playful, but both seems to be dedicated Legion members willing to follow all rules and decisions regardless of what it entails. Hildegard proves this by being willing to execute Lotarus upon discover of her actions while refusing to listen to reason. This again proves her dedication to the rules by refusing to help Rudolph in regards to their safety which is fully allowed by rules allowed to them. Hildegard also likes to have fun in the battle by forcing their opponent to utilize their full potential.
Hildegard was first shown alongside Fagino discussing with Rudolph in castle Azure about Shu and others who have discovered their base in Laibaha. As per Rudolph's order Hildegard along with Fagino reached Laibaha and challenged Shu and others that if they managed to even touch one of them once, then they will not destroy the city. But all of them were unsuccessful to do so even though Hildegard and Fagino were using their shadow forms. When Shu, Marumaro and the knight masters set a distraction, Bouquet tried to touch Hildegard by being invisible but Hildegard easily detected her and made their attempt unsuccesful. After that they destroyed Laibaha city. Hildegard was next shown fighting Shu and Marumaro who were using combination and asked them to meet in a place close to a factory and then assumes dragon form and flew to where other follow him/her. When Shu and the others appear outside of the factory where the town leader arrives. He uses some robots on them as Shu and Marumaro unleash their shadows. They defeat the robots and take the village leader hostage as they enter the factory. Upon entering, they discover slaves working on the furnace and robots. Shu and Marumaro free the slaves and shut down the factory. Minutes later, the slaves start fighting back against the robot soldiers. Shu manages to defeat some of them as Hildegard arrives. Hildegard unleashes his/her dragon in shadow form as Noi merges with Blue Dragon while Bouquet and Hippopotamus merge with Saber Tiger. During the battle, Rudolph instructs Hildegard to leave the area at once. Later Hildegard along with other Investitute Beings began their invasion by attacking towns in different directions. Sometime later Hildegard located Delphinium and attacked her. Though she was using both of her shadows Hildegard injured her heavily causing her to collapse to the ground. As she was going to be killed by him/her, Deathroy used Delphinium's body to summon Chimera and surprise Hildegard with a huge surge of power. He distracted them and escaped with Delphinium. During the invasion General Logi and his troops fight off the winged creatures created by Rudolph that came with Hildegard. Jiro and Xie who were prisoners in Rosekstan's dungeon managed to escape while General Logi and Hildegard unleash their shadows. Jiro manages to take one of the Sleipnir armor and joins General Logi in the fight due to shortage of pilots. General Logi and the armored troops try to take Hildegard down with a powerful attack, but to no avail. Hildegard destroys the armor Jiro was in, causing it to crash to the ground. Jiro tries to use his sword to fight Hildegard who was too quick for him. Hildegard in order to have some fun uses his/her power to awaken Jiro's shadow Minotaur. When Jiro was startled, Hildegard attacked him but Xie pushes Jiro away from Hildegard's attack. With Xie wounded, Jiro decides to use Minotaur to fight Hildegard. Minotaur catches the attack from Hildegard and throws it back at him/her though it was blocked. Both shadows collide their punch with each other until Jiro power ups and Minotaur thrusts Hildegard along with his/her shadow to the ground. When Jiro creates electromagnetic, Minotaur gains a new ability, electric wave to defeat Hildegard and forced him/her to transform into their true form. Before leaving Hildegard warns Jiro that he/she will use their whole strengh next time. Next Hildegard arrives in Z'mey along with Fagino to stop Shu and others from reaching the mast driver. When Shu and others were successful in entering mast driver, it was shown that there was a tracking device at the back of Shu due to which Hildegard and Fagino arrived the area. They combined their attacks and fired at the ship. But it was blocked by Furioso with his brunack sword at the cost of his life. Rudolph calls back Hildegard and Fagino to his castle when the Mast Driver approaches their realm. Later Hildegard watches the battle between Fagino and fully transformed Blue Dragon where Fagino was destroyed in the battle. When Rudolph finds Rottarace guilty of what happened in her encounter with Noi, Rottarace is forced to fight Hildegard. Though Hildegard was shocked and hurt by Rottarace's betrayal he/she obeyed the rules of Investiture Beings of giving punishment to a traitor by attacking Rottarace. Though getting an upper hand, Hildegard was tricked by Rottarace in getting his/her elongated head, arms, and tail stuck. When Rottarace was going to attack Hildegard's body, she was caught by surprise by the emergence of Hildegard's second head who attacks Rottarace. When Hildegard frees himself/herself, Hildegard catches Rottarace by elongating her limbs and electrocutes her thus injuring her very badly. Just as Hildegard was going to finish Rottarace, they were distracted by arrival of Bouquet in Shu's disguise. From other side Shu using Blue Dragon in shadow form attacked them and freed Rottarace in time and gave her to Bouquet. When Rudolph asked Hildegard to join him in battle he/she rejected the offer and decided to be neutral. When Rudolph was battling True Blue Dragon, Noi and combined Saber Tiger alone and winning, Hildegard became sure that Rudolph is sure to win. After Rudolph is destroyed by the combined powers of Shu and shadow wielders, Hildegard finally understood the strength of a human heart and accepted the believes of Rottarace and Noi. After bidding farewell to Shu Noi along with Rottarace and Hildegard who have made Noi their leader, takes their leave to the sky.
Hildegard's dragon form is a long necked green dragon. His/Her appearance looks like that of legendry lochness monster with sharp ridges like wings and a long tail and have two horns levelling his/her head. Like other members of the Legion, he/she is capable of summoning his/her dragon form as a shadow. When he/she releases his/her energy in his/her true form, the sky and the surrounding turns green. He/She can create force field around his/her by creating a large green dragon scale mark. He/She can fire a green beam of powerful blast from his/her mouth which is very lethal and can fire green energy bullets rapidly. He/She can also create green orb of energy discharging electrical sparks which blasts upon impact. Frequently Hildegard is shown to combine their attacks with Fagino as a team. It is shown that Hildegard is originally having two heads one male and one female which he/she ejected during battling Rottarace. It is also shown that he/she has the capability to deliver an attack by surging energy through their limbs and electrocuting the victim.. His/Her unique ability is that he/she can use the power of elasticity where he/she can increase and decrease the length of his/her neck and limbs for combat. His/Her limbs are strong enough to break through solid rocks.