As Twilight Sparkle's assistant, Spike helps keep the library where they live clean and organized, he also assists Twilight in her daily affairs. Little is know about his origin, except that Twilight had to magically hatch him from an egg as part of the entrance exam for the school she attended in Canterlot.
Spike gets along well with all the main six, he also seems to have strong feelings for Rarity.
He is based on the Spike of each generation of My Little Pony since G1, one of which was dubbed a princess despite being male.
As Twilight's assistant, he performs many menial tasks, including taking dictation and cooking. He can also play instruments, sing and dance.
His scales serve as a natural armor that is highly resistant to heat, punctures, and so on. His insides are also tough, as he is able to consume anything from spoiled food to gem stones without ill effects. He can breathe green fire whose primary purpose is to magically transport items (usually letters to Princess Celestia) over great distances. Like other dragons, if his greed is left unchecked, he will physically mature at a rapid rate while becoming increasingly violent.
He is still a baby dragon, and needs lots of rest. Due to this, he has low stamina. He has an unrequited crush on Rarity, which tends to cloud his judgment and even make him physically weak in her presence.
In the series finale of My Little Pony: Friendship is Magic, Spike is much bigger, and serves at Twilight's side.
A Dog and Pony Show
Owl's Well That Ends Well
Secret Of My Excess
Spike At Your Service
Just For Sidekicks
Gauntlet of Fire
The Times They Are A-Changeling
Dungeons and Discords
Father Knows Beast
Sweet and Smoky