I say Kiara. I don't know how she is immature... I'm 12 and when she's a cub she reminds me of me, lol. I like going off and exploring, and chasing butterflies and dragonflies. I've read some of the Six New Adventures books, but he just seems
too much like Simba as a cub. I noticed Kiara looks more like Simba, acts like both and has Nala's expressions. I think Kopa was killed by Zira. I was gonna say he dispersed from the pride but he was gonna be king. I think if he was alive or included in the movie he would be the same age as Nuka.
In the first movie, Mufasa and Nala are the coolest characters.
In the second movie, it would be a tough choice between Vitani, Kovu and Kiara. Nala mellows out in the second one. Maybe being a mother did that

but yeah she lost the attitude.