The main risk is legal action. You could be sued by the copyright holder. This can lead to hefty fines and having to pay damages. Also, your novel might be forced to be withdrawn from publication if it's already been published, which would damage your reputation as an author.
You sure can. Manga is intellectual property, and using it without the right to do so infringes on the copyright. It's important to always respect the rights of the creators and follow the law.
Sure you can. Manga is protected by copyright, and any unauthorized use could land you in legal trouble. This includes reproducing, distributing, or modifying the manga without the owner's consent.
In general, it is not legal without permission. Copyrighted characters are protected intellectual property. If you use them in your novels without proper authorization, you could be facing legal consequences such as a lawsuit for copyright infringement. Publishers are also very cautious about this and usually won't publish works that contain unauthorized use of copyrighted characters.
One benefit is cost - saving. Since they are not copyrighted, you can use them freely without having to pay any royalties. This is great for educators who want to use stories in the classroom without worrying about licensing fees.