All of the characters in the Lion King film served a purpose. Each one had their own individual role throughout the film that associates the story.
For instance, Timon and Pumbaa. They were the ones that raised Cubby Simba through Adulthood. Sarabi, she was the mother of Simba and even though we hardly seen her, she's the one that stood up against Scar, most likely giving all of the other lionesses courage to stand up against him. ^^ You have to acknowledge how brave she truly was.
Zazu, he's the one that kept things smooth for the cubbies. He keeps a eye out on them and reports if there's any dangerous outsiders coming into the Pride Lands. He's one of the key figures that keep the lands safe. Without him, most of the attacks the Outlanders can do will end up in a surprise. >.>
Rafiki, he's the wise baboon or an shaman if you like lol. He was the one who guided Simba back on the path he was meant to follow.
The hyenas were an important aspect for Scar's little group he had going. ^^ Without them, Scar wouldn't be able to start the stampede that he wanted, his plan would've most likely backfired at that point and he'll have to come up with some other brilliant mastermind. Also, the hyenas were the ones that finished Scar off at the end of the film. >.>
Mufasa. lol, no explaination here. >.> We all know how important he is.
Nala... Boy... She is definitely one of the key figures in the first film. She done everything with Simba, got in trouble with Simba and done everything with him. Heck, mid-way through the film, she was the one out of her entire pride to seek help from Scar's devious ruling. ^^ With the whole careless mother thing, Nala is indeed a great mother. I'm most likely sure she helped raise Kiara up to adulthood and kept her well nurished. There's no possible way Simba could've raised Kiara by himself.
Simba... No explaining here. >.>
Kiara, she was important in TLK2SP, mainly because she wanted everything in peace. Even though there was another pride that wanted the Pride Lands for themselves, Kiara knew that they were one, regardless of what they looked like. At the end of the film, you can clearly see that Kiara turned the tables during the fight, she managed to bring peace for everyone.
Kovu, he was a big role in TLK2SP, mainly because he was the bad lion growing up but overtime, he knew the things he was suppose to do were wrong. In the end, he fell in love with Kiara and saught good. We all can clearly see how much he changed and was willing to stop his mother from hurting Simba & Kiara.
Zira, we all know how much of a big role she played. lol. ^^
Well, that pretty much sums up my opinion on things. ^^