10th Elder appears to be an old man with short height and has a bulging stomach. He is mostly bald with some white hair at the back of his head and has a big white moustache shaped downwards. He is seen wearing a suit with a bow tie. He has a somewhat wrinkled face.
In contrast to the 11th Elder who is spoiling for a fight and quick to show his ability, the 10th Elder appears calculating and diplomatic.
He also has a sense of humour, laughing heartily at the thought of the 12th Elder being defeated by a kid like Seira. While fighting with her himself, he taunts her with her relative youth and perceived lack of experience.
He shows a very high opinion of himself and of the other elders. He even posits that humans have surpassed the nobles and dared to proclaim that the nobles of Lukedonia should be the ones to follow orders from the Union.
He seems pretty knowledgeable regarding what fate Seira's father had suffered. He revealed that he was involved in the incident of her father's death.
The 10th & 11th Elders
Rostere comes to Korea to investigate the death of 12th Elder alongside the 11th Elder. They are greeted by Yuri and Bonerre after they disembark the aeroplane. They meet Seira and Regis who arrive as Lukedonia's representatives to attend the meeting with them. Although disbelieving that Lukedonia would send only two children for the meeting, Rostere accepts it and makes a formal introduction. He is further startled to hear when Seira says she was herself engaged in the destruction of the 12th Elder. He asks for explanation from the Nobles but is reprimanded instead. Seira demands of them a proper explanation of the 12th Elder's action in the first place. When Muar reacts at this with his aura and Seira responds with a surpassing aura, Rostere intervenes to stop an anticipated battle. He courteously asks for excuse and promises that the Union will investigate and answer Lukedonia's inquiry.
Afterwards, Rostere and Muar come across Raizel and M-21, inquire after their identities but get no response. So, they consider them as suspects and Muar confronts them. Shockingly to the Elders, Raizel easily blocks Muar's attack which compels both Elders to get serious and prepare for attack. But Frankenstein arrives, accompanied by Seira and Regis, to intervene. As Frankenstein fights and overpowers Muar, Rostere is left impressed at their opponent's skills at using power. Frankenstein's sudden attack on him and Bonerre (which Frankenstein explains as they just 'happened to stand there'), makes him decide to use the opportunity as an excuse and he aims at Frankenstein from behind. However, his attack is answered by Seira who confronts him.
Rostere engages in a battle with Seira. He tries to provoke her but of no avail as Seira simply asks him to cut short and start fighting. Their battle ensues with Seira surprising him with her attacks. Then Seira draws out her Death Scythe and seeing the changed aura of his opponent, Rostere transforms into his full form. His power and ability increases manifold as he blocks her attacks and counters with beams.
At one point, when a losing Muar is about to spill out info to Frankenstein, Rostere aims an attack. Unfortunately, both the attack and Muar are consumed by Frankenstein's Dark Spear aura. Rostere, quite frightened at the prospect of Frankenstein joining Seira to fight him, gets overconfident when Seira declares she'd fight him alone. Although apparently having the upper hand, Rostere is defeated by Seira's Grim Reaper attack.
Rostere attempts to escape in order to survive and plans to hide among normal humans, using them as his living shield. As he jumps to flee, a Blood Field traps him and he crashes onto the ground with the command from a flick of Raizel's finger. Rostere finds it hard to believe that there could be another, yet more powerful being, present there and mistakes Raizel for the Lord of Nobles. Thinking that he'd be the one to take down the noble Lord, he uses up his life force to aim an attack which, when blocked by Raizel, Rostere transforms himself to a living bomb and explodes. However, this suicidal-bomber fails as Raizel encases all the energy and diminishes it.
As the 10th Union Elder, Rostere possesses a high level of authority within the union. He has a powerful "subordinate" working under him but otherwise didn't seem to favour having a large group of people like the 12th Elder did.
Being a modified human, Rostere is very powerful and appears to be more powerful than the Elders after him. In his true form, he is easily able to fight against Seira even while using her Soul Weapon, with him seemingly having the upper hand and her attacks doing little damage against him.
As an Elder, Rostere possesses huge amounts of spiritual energy. He can release yellow energy from his whole body to improve his physical abilities or shoot concentrated energy beam from his palms. These are easily stopped by Seira. He can also shoot power beams from the tip of his horns when he fully transforms. These beams are powerful enough to slice apart buildings.
Rostere has the ability to transform into a rhinoceros-like creature with a long horn in the middle of his forehead. He gains horn-like protrusions on his shoulders, elbows and knees with rings going through them and 4 bony protrusions on each fist. He also gains white hair around his waist(in the shape of a loin cloth) and his ankles. He gains a pair of bracelets around his wrist and a chain around his waist. In this form, his overall abilities are enhanced considerably. His durability is enhanced to the point where he is able to take direct attacks from Seira's Death Scythe and only receives slight scratches in the process. He becomes strong enough to push the young Clan Leader back using only his horns. In finally, he can also become a suicide bomb, by using his whole energy; he turns to yellow energy sphere to explode. This explosion is so strong but easily suppressed by Raizel.