|Information in this article is non-canon|
Rex is blue-green with light-blue skin from underneath his neck, chest, belly, and halfway underneath his tail. He has dark blue eyes and white claws.
He also has a scar over his right eye. It has never completely healed and it stings whenever tears and water comes in contact with it.
Rex is a cheerful kid who takes everything on with an upbeat attitude. He can take everyone on with great determination and when he sets his goals on something, he takes it on with great persistance. He is never afraid to place his complete trust on his friends, but that may become his great weakness someday.
Rex is sometimes too nice for his own good, which has gotten him into tight spots on occasions.
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This article contains plot details about an upcoming episode, book or any other feature of this topic.
Rex's egg was found all alone in a cave by Littlefoot, who also was Rex's first sight of the outside world when he hatched. Littlefoot took pity of the child hatching all alone without his family around that he took him to his grandparents, hoping that they would allow to take him in. While they hesitated at first, Littlefoot's grandparents accepted to take him in and raise him.
The decision started a riot in the Great Valley, but with Littlefoot's determination he proved the child would not harm anyone, and as the child grew with a passive nature the natives felt more at ease while some of them were still skeptic that eventually the inevitable will happen.
Rex grew up to become a gentle "Sharptooth", though his heritage as a "Sharptooth" was unknown to himself until Littlefoot's grandpa revealed it to him, believing he's old enough to know this and restrain himself from becoming one if he ever got the temptation without him knowing he's a "Sharptooth" and believing it was a natural thing as a "Leaf-Eater".
Unfortunately, thanks to the Great Valley invaders, Zill and Bull, Rex succumbed to his "Sharptooth" side and left the Great Valley in fear of hurting his friends.
And thus the A Sharptooth's Heart series begins...
After waking up near the beach, Rex traveled back to the Great Valley on sheer instincts, which he found odd at first. As he crossed a desert he found a Sharptooth family feasting, whom after a 'hard landing' he found out it was none other than Chomper and his family, whom migrate back to the continent every year when the tides go down and the path to his island emerges again in search for food.
Later he has to cross a desert which ended up being chased by a "Sickleclaw", a Velociraptor. After nearing avoiding certain death he befriends the Sickleclaw, known as Yu, whom admits he's been having problems in trying to control his urge to attack everything he sees. After a little musical number to keep the charm of the Land Before Time movies, Yu heads back to his pack. During this Rex has doubts about going back to the valley after what he has done he decides to accompany him and maybe find a place for himself with Yu's pack, the Toviel.
As he tries to get used to life in the Mysterious Beyond Rex makes a promise to try to find his true blood-family, but that is easier said than done. Bull shows up to give Rex a hard time, threatening him that he has to come back to the valley sooner or later, or else he would find a way to make him come back. Rex is worried and afraid about this and goes to sleep...
The next day Yu's pack decides to take down a "Longneck" for food, which they find out the hard way the LongNeck has much more fight in him than they thought. Rex jumps in to help and soon his "Sharptooth" side takes over, leading on slashing up the Longneck's neck wide open. The attack killed the Longneck, but during is collapse its tail strikes Rex down and he falls into a nearby river, which dragged him down a waterfall, where he lost consciousness on shore.
During his state of unconsciousness he dreams a rather random dream, where the ground and dinosaurs are made of clouds. However, he finds a cave where inside he finds Bull abusing and attacking Littlefoot and killed him with a good squeeze to his neck. Rex tried to save him, but it was no use, he just went through him like he was a ghost. Bull later disappears and the cave disappears, leaving Rex outside and come face to face with a 14 foot high humanoid dinosaur with wings, a scar over his right eye. The creature kneels down and permits Rex to place his humanoid paw on the creature's chest, which he realised then and there this dinosaur is Rex's future self, as an adult.
A few hours after waking up the feast on the Longneck has begun, where Rex once again bumps into Chomper. Chomper tells him how his parents and himself join in with Yu's pack every year during their migrations to lend the pack a hand and to benefit from the pack's high ratio of hunting down food. Everything was going swell until Bull once again shows up to make trouble for the pack, and now furious, Rex goes after him. He was led to a cave where a boulder blocked the path, and while Rex tried to move it out of the way he just wasn't too strong. But, thanks to familiar faces from the valley, with their help Rex managed to move the rock off, only to his disappointment to find the cave inside empty. On a good note however, he managed to be reunited with Cera, Ducky, Spike and Petrie, but no Littlefoot in sight.
After catching wind of Litlefoot's situation, Rex introduces the gang to Yu, his pack and a happy reunion again with Chomper, they separate into three teams to find Bull and rescue Littlefoot from his grasp. The team Rex is with, Perie, Cera and Chomper, is found first by a T-Rex Sharptooth, but with a good plan and a little bit of bad luck they managed to show the Sharptooth a thing or two.
Night came and rain started to pour down, but Rex's team still kept on with their search. Without much visual aid, Cera sprains her ankle, which forces Rex to lead them to a nearby cave. Inside the next morning they find Littlefoot, which leads to a good hearted reunion. Their plan to get back with the others backfire when they remember about Cera's foot, which forces Rex to head out on his own to regroup the others, Littlefoot accompanies him though. They find a river where they stop to take a drink, but something attacks them and knocks them both out cold. By the time he woke up he found himself alone, and now scared that Bull might've been the one to find them he runs off to try to track him down, but upon finding another dinosaur his "Sharptooth" instincts overtakes him and attacks the innocent dino. He didn't managed to kill him, but it warped him with guilt. Soon after he bumps into another Sharptooth and managed to escape, but he received a slash over his right eye, which soon will turn into his scar in A Sharptooth's Heart II and future stories.
He returns back to the cave and to his surprise Littlefoot is there, but Cera's wounded, not her ankle but a wound on her back. And to his surprise, they all accuse him about betraying them and attacking Cera. Rex tries to assure them he didn't do anything, but Cera wasn't buying it and tackled him a total of three times, knocking him into the river nearby. Littlefoot goes after him though, believing Rex's innocence.
Soon after a dinosaur resembling Rex, Kumo, shows up and practically challenges Rex. Rex orders Littlefoot to get back to the cave and he stays behind to face Kumo. He admits to Rex he was the one who attacked Cera and jumped Rex earlier before, which angered Rex and forced him to fight. Kumo proved to be the better fighter, with a good slash to Rex's chest and a tail slam to his neck, knocking him out.
When Rex has come to, he finds himself alone again, and after cleaning his wounds in the river he sets out to find his friends again, but along his way he's found by another humanoid dinosaur, Druce. Druce speaks in riddles about Rex's species and later telling him of the war going on between Yu's pack and Bull's pack seconds before disappearing the moment Rex turned his back. Rex goes get to the scene of the war between the two packs, but he spots Bull running from the scene and chooses to go after him, fearing he may be after his friends. Kumo finds him again though, but luckily he wishes not to fight. Together they off to find Bull, though two more humanoid dinosaurs, Pascal and Jaser, find them and help them find Littlefoot and the others.
Together again, they all head to a secret entrance to the valley Jaser and Pascal know of. Rex had the choice to go into the valley again with Littlefoot and the others, but he chose to stay behind until his questions were answered, leading again to the definite separation for now and head back down the mountain to have one final confrontation with Bull, and thanks to a rockslide, Bull was put in his place. Rex and his friends survive to live another day.
And one year later, A Sharptooth's Heart II begins...