[quote="Kopa23"]The directors must have been like ''The main character needs a love interest since all of them have love interests in other Disney movies'', and they created Nala.[/quote]
Surprisingly, that wasn't the case. She was created to be Simba's friend and companion, partner in crime and his voice of reason. The directors decided to focus even more on their friendly relationship and that was one of the major reasons why Mhetoo and his friends were eventually cut out. They wanted her to be his nearest and dearest friend and keep the number of characters to a minimum.
After that they thought about having them falling in love, but that was a very hard subplot to figure out. They didn't want something mushy and couldn't have a long romantic scene at that point of the story. The argument couldn't be avoided for too long. So to keep it interesting they decided to have a funny and short love song

performed by Timon and Pumbaa, but Elton John and Tim Rice were furious!

They went back and forth trying to decide what to do with that subplot and what type of love song it should be until they made a decision:
To cut CYFTLT and the romantic subplot but still having them becoming parents at the end

to fit the circle of life theme. They presented Elton John with preview and he said "This is fantastic, one of the greatest things I've ever seen but what have you done with the love scene?" They explained why it was cut but Elton John was like

So he calls Michael Eisner, the former CEO of The Walt Disney Company, and complained about it saying they were cutting one of the best scenes and songs of the movie. Eisner had a "talk" with the directors about it and they decided (finally!) to keep it under some conditions: it had to feature Timon and Pumbaa at some point, it had to respect the african theme of the movie, it couldn't be too long.
So, that's how Kiara was born.
The End