“This happiness… Please let it last from now and into the future.”
After Yuuki reveals to Zero her feelings, and their issues finally get resolved, their relationship starts to head in the right direction. After a beautiful wedding, the happily married couple head into their future, together, as it should be.
Before long, Yuuki finds herself pregnant. Zero has been preoccupied with work and when he finds out he’s overjoyed. However, the happy news takes a turn for the worst when Yuki miscarries. Yuki starts to blame herself and Zero comforts and assuages her worries, telling her that when they’re ready they can try again.
Zero and Yuuki preoccupy themselves with work and Ai grows impatient, not fully understanding the hardship Zeki has dealt with already through the miscarriage. Eventually, Zeki becomes pregnant again. And again. But each time results in a miscarriage and they are unable to understand why.
So desperate for those answers, they turn to their friend, Aidou, who is obviously knowledgeable on biology since he is making the cure for vampirism. With his help, they discover that each time Yuki is pregnant, the children are twins and consume each other in the womb. They suffer from the infamous twin curse that results in hunters only having one child amongst twins because the strongest twin eats its other half. Worse, the combination of pureblood and hunter genes seems to aggravate the condition and Zero continuously impregnates Yuki with twins as a result of his genetics.
With this knowledge in mind, it takes a toll on Zeki as they start believing their relationship is cursed in more than one way against all odds. Ai harasses Zeki any time she gets to drop hints to her dissatisfaction at not having a sibling. Zeki attempt to evade her interrogation, which leads her to believe something is wrong and eventually she figures out from Aidou that they’re incapable of having children.
Devastation hits Zeki, as both individually guilt and shame themselves into believing they’re not right for each other as they can’t produce a family. However, in light of their bleak turmoil, they don’t give up the fight because it would be like giving up on one another, so with another hopeful try, Yuki finds herself pregnant this time with only one child. This proves to be the secret to successfully carrying a child to term, and Ren is born; their miracle son who was born against all odds.
Thus they have this miraculous moment that they share together, knowing they finally made a family together, despite all the pain and loss they suffered to get to this one moment. Ren is not only the joy of their lives but also symbolizes their difficult relationship, proving that two people, no matter how much hardship they face, can overcome any odds as long as they don’t give up on one another.