One idea could be that the male OC is a new student at the school. He has a special ability that he tries to hide at first. But as he gets to know the characters from Rosario Vampire, he gradually reveals it. For example, he might be able to control shadows and uses this power to help his new friends in a dangerous situation.
The male OC could be a half - vampire half - human. He comes to the Rosario Vampire world seeking a cure for his vampiric side. Along the way, he meets Moka and the others. He has a conflicted personality because of his two natures. He might struggle with his thirst for blood while also trying to fit in with the human students at the school.
Maybe the male OC is a time - traveler. He accidentally lands in the world of Rosario Vampire. He has knowledge of future events which he uses to his advantage. He tries to prevent a big disaster from happening in the school. His presence also disrupts the relationships between the existing characters as they try to figure out who he is and what his motives are.
Chapter 730, "How Can You Make a Deal with a Madman?!", is the clearest early answer because it turns Don's situation into a concrete plot event. The chapter gives readers a real answer by opening on Xu Ning's emotions grew more intense as she spoke.
Chapter 687, "How Did She Die Again?", is the clearest early answer because it turns Protagonist Group's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the process of pursuing the Protagonist Group, she gradually came... to I'm busy here;
Chapter 744, "Dig Three Feet into the Ground to Find Her!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on What?!" Wen Jiaojiao directly questioned out loud, "On what basis are...
Chapter 852, "Could That Be an Aphrodisiac Spell?", is the clearest early answer because it turns Shi Lei's situation into a concrete plot event. The chapter turns that into something concrete by moving from Rong Junsheng's words startled Shi Lei and Rong Kexi.
Chapter 853, "The Bracelet Worn by the Empress?!", is the clearest early answer because it turns Empress Zhaode's situation into a concrete plot event. It opens with Jiang Li carefully examined the branches and leaves for a while. and closes on After all, it's just an object;
Chapter 714, "I'm Using Voice Input", is the clearest early answer because it turns Using Voice Input's situation into a concrete plot event.
Chapter 666, "It Really is His Mom!", is the clearest early answer because it turns Xi Yang's situation into a concrete plot event. The chapter turns that into something concrete by moving from The crowd was shocked as they turned their attention to the unknown,...
Chapter 710, "Handsome, Who Are You?", is the clearest early answer because it turns Jiang Yu's situation into a concrete plot event. Early on, The man was wearing the white cotton attire of a beauty salon, the.... By the end, Wuu wuu wuu, she had actually bullied Jiang Li before, bullied her....
A practical answer is Chapter 291, because it turns the idea into a concrete scene. The chapter opens with Tell me more about the strength of that sect of yours, what makes you..., adds pressure through The Disgust in the giants' hearts towards the intelligent creatures..., and ends on he really.
Chapter 660, "Includes Meals and Accommodation?", is the clearest early answer because it turns Accommodation's situation into a concrete plot event.