Spike has been an assistant to Twilight Sparkle, presumably, ever since he was a hatchling. For her entrance exam in Princess Celestia's School for Gifted Unicorns, Twilight after an initial failure, managed to hatch his egg. For as long as he's known her (as far as we know) he has worked under her: performing tasks like organizing books, taking and sending letters, and other duties around the library.
After Twilight set up shop in Ponyville, his duties expanded. He has been seen working at the Ponyville post office, but is more often found helping Twilight's friends with other tasks, like harvesting apples or digging for gems. He is most willing to help out Rarity, who has a huge crush on.
Spike behaves, obviously, like a little boy. He's "protective" of his masculinity (like any other guy watching this show) and dismisses some activities he gets dragged into as being too girly. As mentioned, he is very supportive of his friends, but this doesn't stop him from getting a laugh out of their misfortunes. Nonetheless, he is respected by the girls.
Spike has a tenancy to sleep-in; something that Twilight understands, since he is still "just a baby". Because of his need to sleep early, Twilight had hired a night-time assistant: an owl named Owlowiscious.
He loves to announce, and provide commentary on, events in town.
Spike was first introduced during the first-generation of My Little Pony toys as an animal companion. He was re-introduced with the release of Friendship is Magic.
Spike isn't in any shows or movies? If that's not right, go add Spike to something!