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 are pestering several examinees prior to the start of the exam. After encountering 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 battle with Heath, Nero decides to come out of Asta's clothes and take 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 is informing several Black Bull members about the emergence of a new dungeon, Nero is sitting on top of Asta's head while Noelle is patting him. Subsequently, he joins Asta, Rack Boltia 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 is heating 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 Rack's suggestion when he proposed to call him Nero.

After walking in the dark for quite sometime, they finally arrive at the inner-side of the dungeon. During the time, Nero remains silence while Rack 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 is having 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 cruising through towards the center of the dungeon until Asta decides to take a detour to save Rack. While the Black Bull members are fighting against Lotus 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 Rack, Nero sees that Asta and Yuno is preparing to fight against another Diamond Kingdom's mage, Mars.

After Asta manages to render Mars unconscious, Nero returns to Asta's side and lands on top of his head. After finally entering 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 that Nero is giving to 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 that Nero was showing to the latter. Seeing that Asta is inside the hidden room, Nero quickly follows and flies passes 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 as the latter awakens after Mimosa is finished healing him and Klaus is suddenly hugging him.

Returning to the Black Bull 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 silence 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 snaps after 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.

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 while Noelle wishes to name him  before agreeing on Rack's suggestion, Nero.