Yuno would be completely shattered. He has always seen Asta as his rival and friend. He would probably blame himself for not being strong enough to protect Asta. His grief might turn into a cold determination to change the world in Asta's memory. Noelle would be heartbroken. She has deep feelings for Asta, and his death would be a huge blow. She might become more withdrawn initially, but then find strength in wanting to carry on Asta's ideals.
Asta's age in the Black Clover story arc changes over time. At the beginning, he's around 15 years old, and as the story goes on, he gets older. The exact age depends on the particular events and chapters.
Well, if Asta is kidnapped in a fanfiction, it could be a plot by some anti - magic group who see Asta's anti - magic as a threat. His sword might be taken away from him too. But Asta being Asta, he would try to fight back even without his sword. His determination would inspire any captives around him to also rise up against their captors. His friends would soon figure out who took him, probably through some detective work by Finral's spatial magic or Gauche's mirror magic.
A potential plot could be Asta's slow descent into darkness. He starts off as a bit more cynical after facing continuous rejections. Then, he meets a group of outcasts who have also been wronged by the system. They convince him that the only way to make things right is to take power by force. Asta begins to use his anti - magic to steal magic from other mages, believing it's a way to level the playing field. As he does this, he becomes more and more feared and is seen as a threat to the kingdom.
One way Asta could turn evil in a fanfiction is if he was constantly ostracized for his lack of magic in a more extreme way. For example, if the Magic Knights not only rejected him but actively bullied him. This could lead to a build - up of anger and resentment. Eventually, he might decide that the system that has shunned him is wrong and needs to be overthrown, leading him down an 'evil' path of using his anti - magic to cause chaos rather than protect.
It could begin during a mission. Asta is separated from the rest of the Black Bulls. While he is alone, a strange magic spell is cast upon him that renders him unconscious. When he wakes up, he finds himself in a dark, damp cell. He realizes he has been kidnapped. The kidnappers could be a group of rogue mages who want to use Asta's anti - magic for their own evil purposes. They start to perform experiments on him to try and understand his power better.