I do not own one piece or Rosario plus vampire. This an idea I've come up with.
He feared this day would come. He knew it in his heart that the charade could not go on forever. Tsukune knew that it was only a matter of time before he was exposed as a human. The student police had taken him to their holding cells to await his "trial". He knew there was no trial and that they were going to kill him. Oddly enough, he didn't fear death. He knew it quite well, for it has visited him twice. One when he was a boy and again 3 years ago. Each time it took one of his brothers from him. He knew that it would eventually come for him to, but not as soon as he expected. His brothers lived dangerous lives and they knew death would not let them become old. They died for what they believed in while he would be killed like a pig. He didn't want to die, not for him, but for his older sibling. He had suffered so much from his brothers pass so tsukune knew that his death would devastate him. He hoped Luffy would forgive him.
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This was not how the headmaster planned for things to go. He fumed as he sat in his office clad in his white robes. He had hoped that by having a human in his school it would encourage the student to get used to living with humans. He didn't expect them to kill him! The headmaster let out a sigh. There was no stopping it, due to some damn old rules he couldn't interfere with the execution, only postpone it. The problem was that because he could only postpone it students were contacting their parents and now he had to deal with them. He was not going to let one his students die, not on his watch!
The headmaster got up and started looking through his files on the students to see what he could find on tsukune Aono. When he got to his file he opened it up and his eyes widened. Since this was a school for monsters the files were enchanted to give all information about the student, even if the student has never told anyone about it. He grinned as a new plan came to him. If his students were going to invite their parents to watch the execution, why not get Tsukune's family to come too? He knew he couldn't contact the boy's parents, just by bumping into the lad's father told him that they were useless. His brother on the other hand was perfect! He could easily save him, he could avoid problems in the future and the none of the blame would go to him! It's perfect!
As he prepared an eternal log pose of the island along with a letter describing the situation, he wondered how the pirate Monkey D Luffy would react to this information.
Thanks for reading. Just an idea I had, since neither of luffy's brothers are related by blood, what's to say tsukune couldn't be his little brother?