Replacement bride.

Basically, it would be an arranged marriage thing (could be collateral for a loan or just a general arranged marriage.) that was supposed to be with MCs older sister but she ran off with someone else. YCs family was offered MC as a replacement for her sister but his family would have given him the final say on whether or not he'd accept the change. When he meets his "bride-to-be" he would find out that not only is she barely of age but that she is a student at YCs school AND in his class. (Maybe the biggest irony is that she is often referred to as the teacher's pet when all she does is help out after school and is actually smart. Aka has earned praise.) The age gap would be something like 10-15 years older than her since she was a "whoopsie" baby compared to her sister who was actually a few years older than him. Romance and/or adventures would go from there. 

My character would be human and her birthday that made her legal would have been a week before this all happens, hence why I refer to her as "barely of age." You can choose between the following characters for this RP:  Melina, Kanna or Caterina. 

To clarify - it would be 75/25 plot to smut. 

I put interspecies just in case you'd like to play an anthro, demon, angel, or whatever. The age gap wouldn't be the only thing in terms of difference. He would also be taller and built bigger than her. (hence the size difference) 

The setting would be modern and we can talk over kinks featured in this and other plots and such. 

Thank you for reading. ^^