Secre Swallowtail

is an Anti-bird who decided to stay alongside Asta after encountering him at the beginning of the Magic Knights Entrance Exam.

Appearance
Nero is a very small bird with a pair of sleek wings, a pair of tails that form an arrow at the tip and a black beak. One of his notable features is his slanted eyes that give Nero a hostile aura around him. Nero's feathers are blue at the back and white at the front, in exception of his facial area where the feathers are red. In addition, Nero also possesses a pair of horns at the side of his head with a comb in the middle that consists of a pair of blue feathers.

Furthermore, the feathers around Nero's neck have a fuzzier texture then the rest of his feathers in which creates the sense of wearing a collar around his neck.

Personality
As an anti-bird, Nero behaves differently around different people where he would be more hostile towards people with low magical power and vice-versa. This particular side of him is noted by his constant annoyance over Asta by pecking his head in which he had never done to anyone else. Additionally, he also has a penchant to use it to get his thoughts across to the latter as well or when he is disagreeing with him.

In contrast to his treatment, Nero is very loyal to Asta as he never leave his side since he met the latter. He also constantly helps the latter on every predicament, such as guiding him through the dungeon or showing him the path to an artifact behind the wall.

Biography
Nero, along with a flock of anti-birds are present when the Magic Knights Entrance Exam is held where they pester several examinees prior to the start of the exam. After he encounters Asta, Nero decides to land on top of his head and sneaks into his attires for a couple of days without revealing himself to the former.

After Asta's fight with Heath Grice ends, Nero decides to come out of Asta's clothes and takes off to observe the Sosshi village from above. He then shifts his attention to an item inside one of the houses. After seeing the item up-close, Nero takes it to Asta and uses it to peck Asta's head in order to wake him up. Nero then tries to hold the item as tight as possible when Asta tries to take it from him with the intention to return it to the villagers.

Afterwards, Nero begins to chase Asta around the village while pecking his head with Noelle Silva following them from behind. He then lands on top of Asta's head while the latter is giving a few words of encouragement to one of the children of the village.

Nero then follows Asta, Magna Swing and Noelle back to the Black Bull headquarters to report about the mission. During that time, Nero meets Vanessa Enoteca whom compliments him to be a cute little creature. Later on, Nero follows Asta, Noelle and Vanessa's venture on Kikka where he remains on close proximity from Asta.

When Yami Sukehiro informs several Black Bull members about the emergence of a new dungeon, Nero sits on top of Asta's head while Noelle pets him. Subsequently, he joins Asta, Luck Voltia, and Noelle to the dungeon when they are assigned on a mission to conquer it. On their way to the inner-side of the dungeon, Asta and Noelle start arguing on the name that they should use to address Nero. The argument heats up when the subject of their argument begins to peck Asta's head to show his disagreement of his suggestion. However, he quickly agrees on Luck's suggestion when he proposed to call him Nero.

After they walk in the dark for quite sometime, they finally arrive at the inner-side of the dungeon. During the time, Nero remains silence while Luck is pulling a prank on Asta and Noelle and leaves almost immediately or when a trap is activated and a member of the Golden Dawn squad, Yuno, saves them from it.

Nero continues to remain silence while Asta and Noelle have a conversation with the Golden Dawn members, whom are Klaus Lunette, Yuno and Mimosa Vermilion. Later on, after both team go their separate way as Asta and Noelle continue to explore the dungeon, Nero finally breaks his silence by showing them a path after he sees that both of the Black Bull members do not know the direction which they need to go.

On contrary, Asta and Noelle are still unable to reach their destination as they are lost in an area with an unusual gravitational field. When Asta tries to ask Nero about it, the latter decides to ignore him and return on being silence while watching both of them.

After a while, Nero, Asta, and Noelle finally find the correct path and cruise through it towards the center of the dungeon until Asta decides to take a detour to save Luck. While the Black Bull members fight against Lotus Whomalt of the Diamond Kingdom, Nero lands on top of Asta's head, but quickly flies away to avoid being a hindrance to them. After Lotus is defeated and manages to escape, Nero follows the Black Bull members to the center of the dungeon. When he arrives along with Noelle and Luck, Nero sees that Asta and Yuno are prepared to fight against another Diamond Kingdom's mage, Mars.

After Asta managed to render Mars unconscious, Nero returns to Asta's side and lands on top of his head. Not long after they enter the dungeon's treasury, Nero quickly tugs Asta towards a wall that has the same locking mechanism as the door to the treasury. He then immediately starts pecking Asta on his head when the latter does not understand the hint, which Nero gave him.

Suddenly, Mars breaks through the treasury's door and manages to incapacitate everyone except Asta and Mimosa. The former then sends Asta through the wall, Nero had shown to the latter. Seeing that Asta is inside the hidden room, Nero quickly followed and flied over him before landing on top of a sword covered with dirt in which located further inside the hidden room. After Asta is finally able to understand Nero's hint, the former takes the sword and resumes his fight.

After Mars is instantaneously defeated by Yuno, Nero remains close to the fainted Asta as all of them rush towards the exit as the dungeon is collapsing. The anti-bird immediately lands on top of Asta's head, the moment the latter is awaken and Klaus suddenly hugs him.

A week after the mission, at the Black Bull's headquarters, Nero joins Asta on a feast that is given to the latter for his performance during the mission. He also remains close when Asta and Noelle need to go to the Magic Knights headquarters at the Noble region to report about the mission. Nero remains silent and does not bother Asta in anyway while the latter is in audience with the Magic Emperor, Julius Nova Chrono, nor when he is attending the ceremony and banquet that honors several members of the Magic Knights.

However, when Asta begins to get annoyed while his comrades are being humiliated by those honorary Knights, Nero decides to take off from Asta's head as he watches the latter confronting them. After the commotion dies down when someone enter the banquet hall to inform the Knights that the Royal Capital has been invaded, Nero returns to Asta's side as the latter joins the Knights' discussion regarding their counterattack strategy. Not long after, the anti-bird follows Asta when the latter decides to move out before the others. Nero remains by Asta's side until they find the man behind the invasion, Rades.

While Asta is fighting against Rades, Nero stays on top of a little girl's head that Asta saved from the latter. Nero remains unruffled even when the little girl is about to be attacked and Noelle steps in to protect her in Asta's place. Afterwards, Nero stays put on top of the little girl's head while they are observing the fight from afar. When Asta returns with the Magic Emperor after being captured by members of the White Night Eye, Nero immediately leaves the little girl's head and greets Asta by pecking is head.

Later on, Nero stays on top of Asta's head as the Black Bull members prepare their departure from the Royal Capital. The anti-bird also remains composed when Yuno sends his farewell to Asta by launching an attack towards him. As they arrive at the Black Bull's headquarters, Nero keeps himself comfortable on top of Asta's head while the Black Bull members are having a conversation around him.

During a blind date, which Asta attends alongside Luck and Finral Roulacase, Nero remains in silence as he observes from the top of Asta's head. However, when Nero notices Sekke Bronzazza is also in a blind date, the anti-bird quickly takes off to the latter's table and intensely observes him. Nero then shifts his attention and observes Asta, alongside Noelle, when the boy is taken into an alley by his date, Rebecca. At then end, Nero returns to Asta's head as the latter asks Rebecca the purpose of a blind date.

Nero then stays by Asta's side when the latter visits Rebecca Scarlet at Nairn. The anti-bird flies away when Gauche Adlai, another Black Bull mage, assaults Asta. After returning to Asta's head, Nero then discovers Noelle who decides to watch them from afar. He approaches the noblewoman and stares at her as Noelle learns about Asta's decision to stay at Rebecca's house. After dinner, Nero goes to sleep next to Asta in one of the bedrooms. However, the moment Gauche sneaks into the bedroom and causes some havoc, Nero quickly flies outside from the hole, which the mirror mage had made. When they realize that the children of Nairn had gone missing, Asta and Gauche, along with Sister Theresa from the church, set forth to pursue them. However, Nero decides to stay at the town alongside Noelle and Rebecca.

Abilities

 * Mana Sensory: Nero possesses the ability to sense mana within living creatures and inanimate objects. Additionally, his abilities allows him to locate magic items from a long distance and through walls. He also could use it to determine the quickest path towards the mana that he is sensing.

Trivia

 * Asta wishes to name him "Toritarou" (トリタロウ) while Noelle wishes to name him "Sylvants Schnauzer" (シルヴァンタス シュナウザー) before agreeing on Luck's suggestion, Nero, which means "black" in Italian.
 * Nero also shares the same name as the real life Roman Emperor, Nero Claudius Caesar Augustus Germanicus.