Cera tends to be somewhat susceptable to jealous feelings, and we can likely agree that jealousy is Not a key to a healthy relationship, of any status. This can be observed most clearly, as Malte stated, in LBT 9 regarding Mo.
However, if you look at when Ali is First introduced to the Gang in LBT 4, before they are aware of her fear of other species, Cera seems quite content, greeting Ali politely. As it becomes clearer that Ali is afraid, Cera tends to play on Ali's fear by stating that "Threehorns can be VERY scary!" Cera is known to pick on the weaker characteristics of her friends (such as in LBT2 when she called littlefoot scared when he wouldn't go after the egg-nappers), so this action is not necessarily one of hostility. Cera just tends to say (and show) what she thinks, and sometimes she is a tad impulsive

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If you look at how Cera and Ali's relationship develops, you can see the acceptance in Cera's behavior. From the moment Cera steps aside to let Ali lie between her and Littlefoot, I think it's safe to say that Cera has lost her negative feelings towards Ali, and there is no sign of jealousy throughout the rest of LBT 4.
The fact that Cera accepts Ali as a good friend, says that she feels that her friends like her for who she is, and she doesn't have to worry about someone else taking her place.
How does this relate to the Cera / Littlefoot crush scenario?
If Cera did have a crush, per se, on Littlefoot, I suspect that she would still show a shimmer of hostility towards Ali, just as you would see in any human relationship. It's not in Cera's character to keep feelings bottled up. I can only think of one example where she sort of tried to:
LBT 7, when the Gang is crossing the vine while searching for Ducky.
Littlefoot: "How are you doing, Cera?"
Cera: *shaking, with a worried voice* "Fine...Doing fine..."
Cera is unskilled at hiding her true feelings, she rarely attempts to, and therefore I don't believe she feels that Ali is of Any threat to her relationship with Littlefoot.
Now, this could mean one of two things:
1) Cera and Littlefoot are on Very good terms, and Cera feels that her bond with littlefoot is special, compared to Ali's bond with him, and Cera has no reason to worry. However, this analysis is very advanced for someone of the age of the Gang. Believe me, I'm 19 and I have trouble working out these things sometimes, lol

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2) Cera genuinely feels friendship and warmth towards Littlefoot, but no more. It's hard to define "Crush" at this age, especially in the case of LBT, where different species are involved. In theory, animals of different species do not look at each other as potential mates, or with feeling of physical "attraction." I will admit the LBT series breaks a number of biological rules, such as, well....the dinosaurs never "relieve" themselves, but that's another topic

but anyways, some rules still must apply to some extent.
All I can figure, is that the Gang is too young to even know that they "can't" be physically attracted to another species, so they sort of pretend that they can, and do. Hence the potential Cera / Littlefoot crush.
However, when it comes to claims that Cera has regarding her Abilities, that's another story completely (LBT 10 - the rock splitting incident in the Great Valley. Enough said

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Cera will admit when she's Really scared, or Really mad, but when it comes to relatively minor situations, she tends to turn the other cheek towards it, and not admit it to her friends. Her friends, of course, can see right past the stories that Cera pushes onto them, further showing that Cera is weak at concealing her true feelings by masking or lying about them.
However, when she is truly feeling bothered, she's not afraid to come out and say it:
For example, in LBT 2: When they fall into the Mysterious Beyond after chasing the egg-nappers, Cera is very sad, and says to the gang, "I wanna go home."
In summary, Cera is a fairly easy character to read; you can tell what mood she is in, simply by her tone of voice, and her actions. As a result, I conclude that she has no negative feelings for Ali by the end of LBT 4.
The Cera/Littlefoot crush is possible, but I do not feel that there is any real basis for it. It's just sort of a warm feeling that Cera has for Littlefoot, for a number of reasons. Perhaps "crush" simply needs to be defined more clearly. I defined "crush" in this writing as, basically, the beginnings of a realtionship that is on a level seperate from a relationship with anyone else; in short, a closer level.
I will say though, that at some level, Cera feels close to Littlefoot, and I'm sure Littlefoot can return this feeling.
I think I'll stop there

: MY apologies if this sounds like rambling, or if I have any non-sensical sentences: It's 2:45 in the morning, but I didn't feel like heading off to bed just yet. I think I will now, though. Night folks! Be back soon
