Cielo is calm, cunning and intelligent. In the military, he was a typical military officer, addressing his soldiers and superiors properly. He considered it disgusting to struggle disgracefully. However, he does make an exception to this if the situation is dire.
He considers Yue to be an excellent pupil, having been her mentor and passing his ideals down to her. He often addresses her as his lady and treats her with more respect compared to the other members.
Sol considers him to be a narcissist. This is shown at both the Fujimino Air Base and Akagi Forest Ruins, where he expresses disappointment and anger at being left alone. He also dislikes being mocked by others, and does not like to surrender while the Chimera still have reserve members.
Before the events of the game, Cielo was known as Seiji Takatsuaka, who served as a military officer, and was friends with Taiki Shinohara. He also served as Yue's mentor. During an unknown battle with the Chimera, he was killed in action, and his body was never recovered. His family was angered by the fact that he was forced to go to battle.
However, he survived the battle, though he became a Chimerized hybrid and changed his name to Cielo.
Cielo is first encountered by the Valkyrie Force at an underground facility, where he sics a large Chimera at them. When it is killed, he takes his leave, promising that they will see him again. He personally attacks the Ichinomiya Military Base with an army of Chimera, abducting Victor Franson. At the Move Shopping Mall, Cielo sends more Chimera, which are defeated by Valkyrie Force. He then leaves with Victor, telling him to meet them at their castle and that he would keep him alive until then.
At the Ise Manufacturing Plant, Cielo turns Victor into a Chimera. He is disappointed that he turned into a normal Chimera, but sics him at the group to sate his destruction. Upon Victor's death, he takes out the Black Garnet, explaining that he used his death as a catalyst, proclaiming that they will no longer need reserve members and sends more Chimera at them. His trap is successful, as he manages to infect one of the members with the Black Garnet as they escape.
Cielo later reappears at the Fujimino Air Base, where he asks Asahi for a solution to a problem he has with Sol. When the two run off, Asahi divides Valkyrie Force in half to pursue them. Cielo patiently bides his time for half of the Force to chase him, but grows impatient and goes to Sol. The Force, who had put Sol as their primary target, are about to apprehend her when he comes and saves her, annoyed by how he had been left alone and leaves with her. He comments on how dangerous it was for them, but is content enough with the results they acquired.
He later reappears at the Akagi Forest Ruins to battle Valkyrie Force. However, as soon as the Force arrives, they immediately leave, as Kana presumed that he and Sol were trying to force the group to use the Valkyrie System and cause their split personalities to appear. Cielo is greatly angered by their supposed cowardice, and makes the irrational decision to personally attack the Ichinomiya Military Base again.
Upon encountering the Force again, he is eager to see them so he can get his revenge for the event at the Akagi Forest Ruins. Upon being defeated once, he shapeshifts into a Chimera, refusing to surrender. Upon being defeated again, he refuses to surrender, not wanting to spill out a hidden plan and prepares to make his escape, only to be shot dead by Taiki.
Taiki later reveals that he killed Cielo because he had grown too arrogant and endangered his plans.