Nathan Mahler

Nathan Mahler serves Diva as her chevalier. He is the last of her chevaliers to appear, and is first introduced in Episode 20.

Origins
As the audience is only shown some of the chevaliers' transformations into chiropterans, Nathan's true identity becomes more of a mystery near the end of the series; even Amshel does not know who he is or what his goals are. It's unclear how or when Nathan became Diva's chevalier or whether he is even her chevalier at all. He says in his first appearance in Episode 20 that he is the only one who has yet to meet her, meaning she had not yet woken from her hibernation cycle when he became her chevalier.

Fans theorize that Nathan is either the chevalier of Saya and Diva's mother or their aunt, and possibly even their biological father. However, the first theory is more popular for multiple reasons, such as the unlikelihood of someone with such blatant homosexual mannerisms fathering children and his immunity to Saya's blood (if one assumes that chevaliers of one queen are not affected by the blood of her offspring). In Episode 48, a conversation between Amshel and Nathan further suggests that Nathan is in fact the chevalier of Saya and Diva's mother:

"Nathan: The world exists as a shining stage for Diva so she can be adored. It was proclaimed even before Diva was born.

Amshel: You talk like you've witnessed it.

Nathan: Actually, I did.

Amshel: Hmm?

Nathan: Years ago, there was Saya and Diva and the mummy that was their mother. What she had then was a faithful chevalier just like her daughter today."

Since Nathan is far more interested in Diva than Saya, and speaks of "History repeating itself", it may be presumed that the queen he originally served was the blue-eyed twin.

In the light novels, it is suggested that the real Nathan Mahler was tracked down and killed by the chevalier of Saya and Diva's mother, who then transformed himself to pose as Nathan and replace him as Diva's chevalier.

Metropolitian Opera House
While watching Diva sing before her final fight with Saya, Nathan firmly tells Amshel that he can have her children, but that he is taking Diva himself. He punctuates this by cracking the glass Amshel is holding merely by looking at it. Amshel himself does not react. This is the second time Nathan threatened one of his fellow chevaliers.

As Diva and Saya fight, Nathan watches from the audience with interest, with Diva's children with him. When Hagi, seemingly having killed Amshel, appears to fight Nathan and aid Saya against Diva, Nathan persuades him not to act by saying that it is a fight between queens.

As Diva crystallizes from the effects of Saya's blood, Nathan brings her her children to see one last time. He laments to Saya how Amshel always treated Diva as an experiment rather than a person and failed to understand that all she wanted was a family, then explains that Diva's blood had lost its poisonous potency against Saya following her pregnancy. He then asks Saya to kill him because he has no more reason to live. Reluctantly, she grants his request. He murmurs, "Good girl," just before she slices him in half, and thanks her while falling to his presumed death into the orchestra pit.

However, despite having been fatally wounded with Saya's blood, Nathan briefly appears afterwards alive and well when several reporters are trying to get information from Van Argeno.

Personality
Unlike the more dour characters of the series, Nathan is the friendliest and most colorful (literally and metaphorically) of all the chevaliers, fitting the stereotype of a flamboyant homosexual. Like Lewis of Red Shield, Nathan provides comic relief within the Cinq Flèches group, usually in the form of annoying James. Nathan repeatedly rubs himself against James and teases him about his "Mama", even stealing James' pocketwatch (which contains a photo of Diva).

Although he is the last to meet her, Nathan is the only chevalier Diva does not lose interest in following her pregnancy. He seems to take a paternal attitude towards her and fusses over her appearance. Nathan appears to care a great deal about Diva, as he is willing to let Amshel take her children in exchange for her. He is visibly saddened by her death. Out of all Diva's chevaliers, Nathan may have understood her the most.

Nathan neither attacks Red Shield nor defends any of his fellow chevaliers from attack when involved in a battle. Despite this, he does show hints of compassion towards his fellow chevaliers, including Hagi. For example, although he and Karl have no direct contact with each other in any scenes they are shown in together, Nathan asks the other chevaliers "Are you sad that our brother is dead?" following Karl's death. Nathan also goes out of his way to help Solomon, such as checking up on him after he is rejected by Saya in New York and freeing him and giving him the rest of his clothes after Solomon is imprisoned for trying to kill Diva. The exception to this is Amshel but this is not seen until the very end of the series: Amshel tries subtly to provoke and antagonize Nathan while at the same time seeming wary of him. Nathan shows no emotion to Amshel's apparent demise and criticizes him for never having understood Diva.

Despite his bright and cheerful personality, Nathan can be deeply threatening when needs be. He rarely (if ever) gives orders, but none of the other chevaliers question him. Nathan quickly drops the flamboyant façade altogether when confronting Amshel. It's quite possible the flamboyancy is an act, or that the situation for him is a very serious affair.

Powers and abilities
Nathan's exact powers are unclear. He can either move so fast that he cannot be seen with the naked eye, or he can change his form to trick others into not being able to see him. He also appears to have a form of telekinesis stronger than the other chevaliers, seemingly able to shatter glass with his voice.

Manga
Nathan is not hinted to be connection to Saya and Diva's mother or aunt.

Diva tries to kill him without hesitation when they attack the Red Shield boat. While Solomon is able to save him, the damage has been done: her attempt on his life causes a rift between them and he begins to harbor bottled feelings of disdain, rage, and hatred towards Diva, which only increases when he sees her indifference over James' death.

During Saya and Diva's last battle, Nathan finally snaps in response to Hagi's taunts. When it seems that the sisters might reconcile, he stabs through Saya into Diva, causing Diva to crystallize and shatter. He is killed by an enraged Saya.

Trivia

 * Because of his grandiose nature and the fact that he is openly gay, the fans have playfully dubbed Nathan the "third queen."
 * Excluding a scene in the fourth opening, Nathan is the only chiropteran who never fights against Saya or Red Shield. On one occasion, he even actively defends Saya from attack by James.