I think Kovu is the other half of Kiara. I know it sounds crazy,but I will explain:
When Kiara was a cub she said:
I'm not just a princess you know.That's only a half of who I am! Then Pumbaa said:
"And who is the other half?" Kiara answered with a
"Ehm...well I...ehmm" Which meant that she didn't know! But when she fell in love with Kovu.....
After Simba had exiled Kovu from the Pridelands Kiara ran away to find Kovu,and she stopped in a waterhole and looked at her reflection. And this is what she saw:

Only a half of her!
And after Love Will Find A Way Kovu and Kiara sat and looked at the reflection they had together,and Kovu said:
"Look,we are one"
Short after that moment,Kiara got big eyes and said "One...?" It was like she understood that Kovu was a part of her.
I know if you maybe don't get why I am trying to say,but that's because this is my first post and I'm very bad to explain things,especially in English xD