Forgive the word vomit ahead, but since I’m working on a fic that revolves around Ali, I have many thoughts about her that came up while writing her and surprisingly only a little has anything to do with romance.
As others have noted, Ali is a bit of a mischief maker, with biting Littlefoot's tail, making faces, and tagging him it out of nowhere. It seems she has a taste for these games of mischief and it's pretty cute.
She can also be pretty brave. From being willing to accompany Littlefoot on the journey to cure Grandpa Longneck, to having to go back to the Great Valley on her own to get help for Littlefoot, and saving Cera by riding a giant crocodile, she has done some pretty bold things.
What is also interesting is what she doesn't say. When Littlefoot expresses disappointment his friends went home after he played with Ali, she vaguely says "I guess" while looking away. When Ali accepts she'll accompany Littlefoot on the Night Flower mission, she dissuades him from bringing the others along, saying they would slow them down. Ali doesn't want to have his friends around but she’s too polite to outright say it, so she speaks vaguely and attempts to distract Littlefoot to get off the subject. It seems she has been taught to hold her tongue when she has something nice to say. Though it doesn’t stick with her through all her interactions, it kicks into effect when Littlefoot’s friends come up.
I agree implicit prejudice might be the source of Ali’s nervousness but there’s another cause that’s overlooked. Her herd is migratory and comes from the Land of Mists. There are a lot of dangers in both arenas, so she probably had it drilled into her to not approach different kinds. They might be sneaky predators or potentially hostile or manipulative herds, so she can only interact with other longnecks. At least their ways are more familiar to the Old One’s herds, though I wouldn’t be surprised if she was told to have some caution for stranger longnecks as well.
So when Ali ran into Cera and the others, she probably knows logically Cera and the others wouldn’t really hurt her but those lessons of caution made her automatically freeze up in fear. She couldn’t really shake it off until Littlefoot got trapped in the cave. When presented with the choice of either letting Littlefoot die on the other side or getting help from Cera and the others, she got some perspective on what is really important in the situation and pushed her fears aside to retrieve their help. How she interacts with them thereafter indicates she was trying to make up for her behavior and when Cera accepts her friendship, she must have felt really happy.
As can be seen when Ali wonders if there’s anything she and her mother could do about Grandpa Longneck’s ailment, she does sincerely want to help Littlefoot. Even when she hesitated Littlefoot’s request, it seemed to be more about attempting to get over the caution her herd drilled into her. She tries to help him with what she knows, but it’s interesting how Ali doesn’t reveal the true extent of her knowledge until the expedition comes across a snag. I don’t believe it’s out of malice. She probably thought she remembered enough they could wing it and discover other helpful details along the way. She doesn’t want to curse their luck by admitting aloud her knowledge gaps.
In concern with Ali’s depiction in the TV series, the other posters do make some good points. She only did know the gang for three or four day while she probably was in Rhett’s presence for months at least. As for why Ali believed his lies…I kind of think she went through what Doctor Who’s Donna went through after her initial one-off appearance. Though it was only for a couple days, Ali went through an intense adventure with some fellow youths that changed her views of the world and she was happy with. The intense bonds shared through such an experience is probably something she discovered she loved and treasured deeply.
And then the day after she leaves the valley…she’s back in the same routine. The herd must travel from point A to point B, she shouldn’t wander far, be cautious of strangers, etc. She tries to make friends with what fellow youths might be there but she has a hard time connecting with them while remaining true to herself. At some point, she probably became lonely and began making compromises so she can make some friends. So when Rhett came around with his tall tales and an interest in making friends, Ali might have willed herself to think that her adventures with Littlefoot’s gang prove that other youths battling enemies isn’t that farfetched.
It’s clear from scenes in the episode Ali isn’t always comfortable with how he takes charge, but probably reasoned to make due with what friends you can get. Hopefully after the episode, when all the facts become clear, she’ll relearn to be bold and that there are some things she shouldn’t compromise about herself. She and Rhett might still be friends, but maybe
she'll be the one leading the games and adventures they get up to. That’s what I hope for, anyway.
I’m aware Ali was likely intended to be a love interest for Littlefoot. Nevertheless, I found Ali and Cera’s interactions the most interesting, to the point to I kind of ship those two instead. I’m not much of a shipper, but when I am, I’m usually biased toward the queer side of things
