Here are the descriptions of the various fighter/sacrifice pairs in Loveless.
Seimei Aoyagi
Sacrifice
Age: 18 or 19 years old
Seimei was Rituska’s older brother who was believed to be murdered when he was 17. His body was found burned and placed at Ritsuka’s desk at school. While alive, Seimei always protected and defended Ritsuka from their mother. Seimei cared for Ritsuka very much, and was adored by their mother. Seimei had a secret life as Beloved that none of his family knew about.
Soubi Agastsuma
Blank Fighter
Age: 21 or 22 years old
Soubi was assigned to Seimei by Ritsu. As a child, Soubi was orphaned and then taken in by Ritsu. At age 17, Soubi was made to become Beloved's (Seimei's) fighter. Soubi was completely devoted to Seimei and would do anything he wished. Seimei had asked Soubi to look after Ritsuka is he (Seimei) were to disappear, so when Seimei appeared to have been "murdered", Soubi sought out Ritsuka. Soubi seems to care very much for Ritsuka.
Nisei Akame
Fighter
Age: ?
Nisei is said to be Seimei's true fighter unit and follows all of Seimei's orders. Nisei is cruel, malicious, and manipulative.
"One without love."
Ritsuka Aoyagi
Sacrifice
Age: 12 years old
Ritsuka is emotionally scared because of his older brother Seimei's death two years ago. Seimei's death had also led to Ritsuka's mother becoming physically and verbally abusive; she often tells Ritsuka that he's a different person since Seimei died. Ritsuka changed when Seimei died, mostly by withdrawing into himself. When Soubi comes into Ritsuka's life, he takes a liking to him solely because he was a friend of Seimei’s. Ritsuka once struggled with whether Soubi could be trusted, but as they spend more time together, Ritsuka warms up to Soubi. Ritsuka has a desperate need to love and be loved, but has a hard time trusting others. After meeting Soubi, Ritsuka finds out he is the sacrifice of fighter unit Loveless, but doesn’t know where the fighter Loveless is. Ritsuka allows Soubi to be his fighter unit and is constantly trying to unravel the mysteries surrounding his brother Seimei.
"Our name is Zero. We are the beginning and the end. We are everything and nothing. We shall give you pain and suffering. We shall send your existence back to nothing."
Yoji Sagan
Sacrifice
Age: 12 years old
Yoji is the twin brother of Natsuo.
Natsuo Sagan
Fighter
Age: 12 years old
Twin brother of Yoji. When Natsuo was a child he gouged out his eye to see if it was true that he couldn't feel pain.
Yoji and Natsuo have similar personalities. They were both raised by Nagisa to be ultimate fighters and were created with the inability to feel pain. While this makes them "strong" fighters, it also is a great weakness, being that their bodies could die without them realizing it (from wounds, the cold, etc.) Natsuo and Yoji are mischievous, obnoxious, curious, and sometimes seemly cold or cruel. They don't have a good sense of human emotions, but become very fond of Ritsuka and Soubi. They also have strong feelings for Nagisa and want her praise and approval. Nagisa is often very harsh with the twins, sometimes forbidding them to come home. When they can't return home they stay at Soubi's place.
"We are of the same soul. We share the same name, Breathless; an intensity that will take your breath away."
Midori
Sacrifice
Age: 12 years old
Ai
Fighter
Age: 12 years old
Breathless is a pair that is made up of Ai and Midori. Their names are on the palms of one of their hands. Ai is a girl, and Midori is a boy. They are in the same grade as Ritsuka and are sent after Ritsuka to bring him back to their boss Septimal Moon. They are disgusted that Soubi fights with the Loveless unit, even though he belongs to the Beloved unit. Ai and Midori are very close and don’t like to be separated. They tell Ritsuka not to trust Soubi.
"Our name is Sleepless. We share one name. See the darkness of a sleepless night!"
Ginka
Sacrifice
Age: ?
Kinka
Fighter
Age: ?
Sleepless uses spells that envelope their opponent in darkness.
Yamato Nakano
Sacrifice
Age: ?
Yamato is very sociable.
Koya Sakagami
Fighter
Age: ?
Koya is soft spoken and wears fake cat ears since her friends from school still have theirs.
Yamato and Koya seem to be an earlier version of Zeros since they are older than Natsuo and Yoji. Their names are on their chests above their breasts. They were introduced to each other when they were younger, and Nagisa told Yamato that Koya was in charge of her. Yamato and Koya became close very quickly and are now loyal companions. They attend different schools in the outside world, and When Koya and Yamato go to fight Soubi and Ritsuka, they end up being challenged by Natsuo and Yoji, and they use a spell to end the battle quickly, which temporarily severs the bond between fighter and sacrifice. Due to this (or maybe some other circumstance) Yamato begins to lose her Zero name on her body and it soon disappears; which means she is able to feel pain. Koya finds this out during their second battle with Soubi and doesn't want Yamato to be put through so much pain, so she forfeits the battle and therefore can never fight again. They tell Nagisa that they're never coming back, so that they can try to live a normal life together.
"We are fearless. And that's our power. We aren't afraid of anything."
Mimuro
Sacrifice
Age: 18 years old
Mimuro is loyal to Nisei and does whatever Nisei asks of him. He worries about Nisei's relationship with Seimei.
Mei Hotarubi
Fighter
Age: ?
Mei looks to be about grade school age. She has a crush on Mimuro and dresses as a boy because she believes Mimuro prefers boys over girls. She doesn't like the way Nisei treats Mimuro.
Fearless was first sent to fight Loveless as a distraction for Nisei. Mimuro wants their style of fighting to focus on the offensive, rather than defense. But Mei uses defense spells, because she doesn't want Mimuro to be hurt. Fearless has their names on the backs of one of their hands.
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Hideo Yamamoto
Sacrifice
Age: ?
Hideo was traumatized when his fighter Meiko was taken from him. He seems to hate Meiko's replacement, Yurio.
Meiko
Fighter
Age: ?
Meiko was the original fighter unit for Bloodless. Her name was on her shoulder. Nisei had Meiko destroyed so Hideo could be paired with Yurio.
Yurio
Fighter
Age: ?
Yurio is eccentric and prefers to be addressed as "The Great Yurio" (Yurio-sama in Japanese). She knows that Hideo hates her, but seems to want to be accepted. Hideo put Bloodless's name on Yurio's cheek.
Bloodless is called on by Nisei to hold Kio hostage from Soubi and Ritsuka. Bloodless fighting strategy is to work their way into their opponent's mind to destroy them through fear and illusions. They are called Bloodless because they can destroy their opponent without shedding any blood.
"On days without a moon, no one can defeat us."
Mikado Gomon
Sacrifice
Age: 14 years old
Mikado was once very close with Seimei. Mikado now hates Seimei because Seimei had Nisei rape Mikado (she still had her cat ears) and cut off her long hair. When Mikado asked Seimei why he had Nisei do that to her, Seimei told her there was no reason behind it. Mikado find's that unacceptable.
Tokino Fujiwara
Fighter
Age: ?
Tokino is very close with Mikado and hates Seimei.
Moonless are strongest when the moon is hidden from the sky. They are out to find Seimei.
"No one can conceal their true face from me. Because I’m faceless."
Hatsuko Shigemori
?
Age: ?
Keiji Shigemori
?
Age: ?
Writes novels.
Faceless is a sibling pair. Since they were children, it was easy for them to blend in with people and go unnoticed. They have a strong sense of what people are feeling. They were hired by Moonless to find Seimei. Hatsuko meets with Nisei and gives him information about Ritsuka for Seimei.