Crash and Eddie are supporting characters of the Ice Age franchise. They are two mischievous opossums and Ellie's adoptive brothers.
Why They Can Fly
They are basically Bart Simpson splet in (or multiplied by) two.
Like with all the characters in the Ice Age franchise, their designs are great and beautiful to look at. Although they are very close lookalikes, it is also relatively easy to tell who's who: Crash has a flatter nose and has blue eyes while Eddie has a pointed nose and brown eyes. Eddie also has some hair between his eyes.
Their foolish antics can be hilarious, like when they drive Sid and Diego nuts by playing Whac-A-Mole with them, as well as continually teasing them by saying I felt some brise in that one, Hey, ugly!, Smile!, etc., when they first meet, when they roll with a log downhill, when they mock Alvin and the Chipmunks by singing a bit of "Christmas Don't Be Late" with high-pitched voices after they are temporarily affected by the laughing gas in the Chasm of Death and whenever they play dead to escape danger or childishly fight each other like twin brothers normally do. They also know they are foolish and are proud of that.
The iconic I believe I can fly scene.
Their chemistry with Ellie and Buck is hilarious and amazing.
They had their moment to shine in Dawn of the Dinosaurs when they along with Buck rescued Sid from falling into the lava.
Their chemistry with Manny, Sid and Diego is also neat. While they don't get along at first, with the main trio finding the possums' foolish behavior annoying, along with the possums not trusting Manny at first, initially thinking he can hurt their big sister somehow, as well as their first "Whac-A-Mole" encounter with Sid and Diego not exactly leaving the best first impression, they eventually become attatched to the trio, as evidenced at the end of The Meltdown when they were heartbroken to say goodbye to them, and were more than happy when Manny finally let go of his past and expressed his feelings to Ellie, with Crash and Eddie being more than happy to have Manny as their brother-in-law upon noticing how much he truly cares about their sister. The fondness is reciprocated by the trio, as evidenced when Diego, who initially wanted to eat them for their mischievous antics, later willingly lets them ride on his back, and even smiles at them.
Seann William Scott and Josh Peck do amazing jobs voicing them, and making their voiced different from each other, with Eddie sounding more shy and timid than Crash. Their replacements in Buck Wild, Vincent Tong and Aaron Harris, respectively, also do a pretty good job at sounding like them.
In The Great Egg-Scapade, they created April Fools with very clever ideas for pranks, even though they all hilariously backfired on them, until one of them captured Squint.
Despite their foolish behavior, they care for their family, especially their sister Ellie. Not only did they accept her as their sister with no hesitation when their mother chose to adopt and raise her (which is also a positive message to adoptive families), but they are willing to put their lives on the line for her despite claiming that, for possums, bravery is dumb.
When they get trapped in a cave due to falling rocks alongside, they refuse to leave their big sister behind, only exiting the cave when she forces them out, causing them to immediately rush to seek for help.
When Manny couldn't tell Ellie how he felt about her for still thinking he is replacing his previous family, Crash and Eddie, along with their sister, were heartbroken to say goodbye to him, Sid and Diego, and when Manny finally decided to let go of his past and be with Ellie, Crash and Eddie were more than happy for their sister finding love, even crying out of joy, and willingly accpedted Manny as their brother-in-law.
When Sid was upset Manny did not include him in his special family ice-mobile, Crash and Eddie offered to place him in theirs.
When Buck tells them to come with him to save Sid, they are hesitant to leave Ellie at first and before going off with the weasel, they remind Manny to take care of their sister.
While they claim to not like Sid in the gas scene, they were likely joking or it was an effect of the gas, as earlier on, Eddie was worrying that Rudy may find Sid.
In A Mammoth Christmas, when Manny tells Sid he is on Santa Claus' Naughty List after he accidentally breaks Manny's Christmas Rock, making Sid cry, Crash and Eddie attempt to comfort Sid, and join him and Peaches on a quest to find Santa Claus so they can both prove to Manny that he is a real and take Sid (along with Crash and Eddie themselves) out of the Naughty Life.
They know when to take things seriously and they never joke or play with serious matters or high stakes, like the flood from the second movie or the hidden underground world of dinosaurs in the third movie.
Although they start off as unlikeable in The Ice Age Adventures of Buck Wild by acting spoiled towards Ellie, they get likeable after Buck is is captured by Orson so they, Zee and Momma Dino can escape, as they start to think about the consequences of their stupidity and even blame themselves for Buck being captured, which is a huge improvement over their portrayal in Continental Drift and Collision Course, where they didn't even care about their friends possibly dying. They decide to train with Zee to find the bravery inside them, which is also a pretty good message. It's also interesting to see Crash and Eddie having character development after spending all the other sequels being just comic reliefs, even though they were likable and funny before Continental Drift.
They were horribly flanderized in Continental Drift and Collision Course from intelligent and crafty pranksters to obnoxious and brainless idiots that never take anything seriously and don't care about anyone's safety, not even their own. Although they slightly improve on Adventures of Buck Wild, that's not saying much, as they still act spoiled towards Ellie and the trio, though they do become likable later on.
Even before their flanderizations, while still likable and funny, they can get annoying from time to time.
Their personalities aren't very different from each other, unlike other comedic duos like Chip and Dale where, while they are both mischievous, Chip is the stubborn, slightly bossy smart one and Dale is the impulsive, lazy and dim-witted one. While Crash and Eddie do occasionally fight each other, they think the same, they act the same to the point they could have just be one character, and nothing would significantly change in the movies.