Seemingly from nowhere, ex-Akumukage ZK and new ninja Gatomon seem to have had children...
Not in the way you think.
Their bundles of joy hail from different time periods about Vescrutia and just so happen to have fallen into the arms of the loving (yet mismatched) pair of ZK and Gatomon. Terra's story begins long ago in the ancient times, before Ninjutsu was based in the user. The techniques were drawn from nature and the more in tune the user was with their surroundings, the more easily they could use their chakra. Terra grew from unkown roots, but ended up locked in a chamber far below what is now known as the Alps of Chaos. Though her history is unknown, ZK knew a great strength in her so he ventured with Zeik to release her from her solitary bondage. Succeeding in doing so, he took her back to his home, where she was kidnapped by Chase Young and she proceeded to work with him to exact vengeance on the world that condemned her for so long.
A team of Nightmare ninja took on Chase and she was returned to ZK where he saw that her current state of mental affairs would do him no good so the only solution would be to re-raise her as his own. He the Sands of Time to revert Terra back to a childlike state of no more than 10 years old. From there, Gatomon proceeded to build her into a young lady (even though she didn't agree with the idea at first).
Now she's still emotional, but her stability is more stable and she has begun life in the Chaos.
Nero on the other hand is ZK's real son from the future.
Evidently, in the future, the world of the dead will wage war on the world of the living and Vescrutia is the only thing standing in their way to getting there. That being said, they rush the Vescrutia villages and quickly overrun them. Even the strongest planet destroying ninja are meeting their matches in galactic size battles.
Nero meets with seemingly nice girl in his time, also no more than 10 years old the both of them. As their relationship unfolds, it is revealed that she is a vampire. Nero seems not to mind the situation since they had no problems before.
His turning point in life comes when she needs his blood to survive an attack by the undead-ish legions. He holds his ground and protects her until ZK appears and gets them both out of harm's way. The vampire girl still needs blood so Nero offers his readily. She bites him and he makes the change into a vampire himself.
The unfortunate thing is that in Nero's near future, Arceus, the Creation Pokemon, orders Palkia and Dialga to handle the situation between Vescrutia and they do as they see fit.
Dialga and Palkia go to Vescrutia and begin blasting everything in their path, including those defending Vescrutia. Hyper Beams and Earthquakes, Thunders and Overheats, massive collateral damage is done.
It just so happens that Nero gets caught in the crossfire and through a Roar of Time, he gets time blasted conveniently back into this time period.
Quite convenient.
Now, he stays with his past father, learning from him to go back to the future and try to help more with the battle.
Being a vampire now, he ages more quickly than he would normally so he now has the look, demeanor, and intelligence of a 14 year old spaz.
Together, Nero and Terra are quite the pair with their original techniques and tag teams. Their battle styles are effective together and alone.
For now, they live the easy life of sprout ninjalings, but their plot thickens as it grows more urgent that Nero return to the future to help his father and the others.
They are siblings at heart, but their minds are at two different places...
How far will they go for each other?
Terra
Nero