Disclaimer: I do not own Black Clover, or any of its associated characters and lore. That right belongs to Yūki Tabata.
While training for the Magic Knights Entrance Exam, Asta had a thought on how to use his Anti-Magic imbued sword for something other than cutting magic and blunting wood. Getting a rough idea on how to make eyeglasses, he used his sword to aid in forging the lenses and the frame itself at the Demon skeleton on the outskirts of his home of Hage Village.
The concept behind the glasses was for Asta to use them to see through magic just as he needed his sword to cut it, which he figured would be useful if he dealt with people using illusions and transformations after he became a Magic Knight. Having completed his crafting after over an hour at the skeleton, he was about to head back to the orphanage church where he lived to see if his friend Yuno could help him test them out with his own magic while wearing the glasses in front of his eyes, which was when he noticed something odd with the nearby statue of the First Wizard King that caught his attention.
"Huh, that's weird. "Asta commented as he approached the statue closer. With his glasses, he saw that in place of a statue made of stone, there was a flesh and blood, yet completely still, version of the First Wizard King himself just as there was the statue still without the glasses. Although Asta was not known for his tact, it did not take him long to realize the circumstances. "The Wizard King was turned into the statue!" He could not help but exclaim. Wondering if his Anti-Magic could break the Wizard King out of his situation, Asta quickly summoned his sword when he then heard a girl's voice.
"Even if it did work with the Demon-Slayer Sword, I wouldn't allow it." The voice sternly said to a spooked Asta. "As much as I would love to see the Prince again, there's still a possibility that something will go wrong using Anti-Magic."
"Uh, who's talking to me?!" Asta yelled in alarm.
"Quit yelling and look behind you, boy." The voice instructed harshly as Asta did so to find a floating girl about two years older than him in a black dress to match her hair with horns upon her head.
"Who are you?" Asta could asked a bit calmer yet no less curious.
"Secre Swallowtail, noblewoman in service of Prince Lemiel Silvamillion Clover, the one you refer to as the First Wizard King." She introduced herself to Asta and quickly added in anticipation of his next question. "The reason I myself am still alive as well as floating is because my overuse of Forbidden Magic led to my transformation into an anti-bird with a prolonged lifespan in addition."
"Huh?" Asta commented as he removed his glasses to see that Secre's present form was indeed a black, white, and red feathered bird. "That's actually kind of awesome." He said to Secre partly to be nice and partly because it was in a sense to him just that.
Secre could not resist scoffing at that. "It probably would be if I could change back on my own power, but I'll have to rely on the same means to free Prince Lemiel instead and that won't happen until he's really needed."
"Can I ask what happened to the both of you?" Asta asked in concern while looking back at the petrified Wizard King.
Sighing, Secre replied. "It's complicated, but since you seem to be uniquely gifted, I'll let you in on it in time in exchange for letting me accompany you on your future career as a Magic Knight, but keep my true identity a secret."
Showing a smile, Asta responded. "Sure, Secre. I'm Asta, by the way."
"I guess it's nice to meet you then, Asta." Secre said as she then laid herself on Asta's head in her anti-bird form.
"Nice to meet you, too." Asta said back as he then went back to the orphanage church with his new companion.
At first, I thought about making this one of those depictions of Asta using Anti-Magic from the start, but then I had a thought of what constructive uses Anti-Magic items could be used for, so here you go. I also wanted to make it a bit more romantic, but I figured that so long as Secre is stuck as Nero the Anti-Bird, that wouldn't happen for a while. Hope you liked it, still, and be on the lookout for two more Black Clover one-shots at least to go involving the Azure Deer Magic Knight Squad and the Heart Kingdom.