Who is he getting married to?

Well, Rio, the sister of Ethan, for one. As said in her profile, often she will appear out of nowhere into Zero's arms or lap right in front of Juliet and Mindy or use her body, charm, or her family's resources to try and win his heart, much to the descendant of Danny's dismay.

Then there's Juliet. She has the intendency to share things with her cousin at times, as she always has the plans to marry him one day. She's also very possessive of Zero.

And of course, who could not forget Mindy? She would prefer to be 100 feet away from her uncle rather than be separated from Zero under his rules. She would do anything in her power to be with Zero forever.

Then, of course, there's Sia, the Wolfess: http://www.furaffinity.net/view/10089603/

She is the descendant of Luna and Jack who, along with her brother, just moved to the battle school where Team Zero attends to. It is unknown who her mother and father are, though she does seem to develop an interest in Zero. She usually spends her time doing origami when Dark Satan is not attacking. She also dreams of being the bride of Zero one day.

So...yeah. He's getting married to four girls. Also, I was planning on basing Zero's story on Castlevania...ever heard of that franchise?