Spike is a small purple and green baby dragon with green eyes who is Twilight Sparkle's assistant and roommate. Spike also handles communications between Twilight and Princess Celestia, burning Twilight's written messages with his fiery breath to send them and then belches up the scrolls Celestia sends back. He frequently displays a lightly sardonic personality and often finds humor in others' misfortunes, but he is rarely malicious and can always be counted on to help Twilight and her friends when they need him.
Spike has a crush on Rarity, but she was not aware of his affections until the second season. In "The Cutie Mark Chronicles", it is shown how Twilight hatched Spike from an egg as part of an admission exam for a school of magic run by Celestia, gaining her cutie mark and a chance to study under Celestia's direct guidance. In "Dragon Quest", after the incidents with mature dragons, he adopts a baby phoenix as a pet and names it Peewee, but in "Just for Sidekicks", a picture-framed photo revealed that Spike brought him back to his family.
Spike is slightly self-centered, and can become jealous on some occasions. In the series, Spike reveals several times that he is highly fond of Twilight Sparkle, who he has assisted and lived with his whole life. His worst nightmare is seen to be Twilight ordering him to leave her. This is exemplified in the episode "Owl's Well That Ends Well", where Spike becomes jealous of Owlowiscious, Twilight's pet owl who assists her with things Spike can't. He makes a decoy dead mouse, and blames it on Owlowiscious. But Twilight knew he was faking it the minute she walked in. She scolds Spike, and walks upstairs. Spike then thinks she hates him, and runs off into the Everfree Forest. He finds a cave loaded with gemstones and crystals (Spike's favorite snack), which it turns out that it all belonged to another dragon. The dragon attacks Spike, but Owlowiscious saves his life by creating a diversion. Spike runs to Twilight, sobbing and regretting ever running away. She forgives him, and explains to him that he helps her in the day, but Owlowiscious has to help her at night. Spike's personality does change slightly after that, being more careful with his temper. He starts to show sympathy to others, teases less frequently, but still continues adding humor with Pinkie in the episodes.
In My Little Pony: Equestria Girls, Spike turns into a dog when he follows Twilight into the alternate world.