Santa Claus is a jolly toy maker who delivers gifts to children worldwide on every Christmas Eve. He first appeared in the Oh Yeah! Cartoons short, "The Temp".
The spirit of Christmas appears in The Fairly OddParents a number of times, his appearance almost always changes in each episode due to animation, or changes to his character design. He remains more or less the same though, the jolly and nice Santa Claus, but a few appearances he seems a little bit meaner or is implied to be secretly evil.
During normal days, Santa Claus appears as a thin, professional looking business man who runs a large factory in the North Pole. He wears a gray business suit, has a trimmed beard, and has sunglasses over his eyes. When in this form, Santa Claus is completely normal and powerless.
When granted fairy magic every Christmas, Santa Claus's beard grows, his clothes transform into his iconic red garb, and he becomes fatter. He gains the power to create toys with his magic, as well as use his magic to defend himself although this will drain his abilities quick.
Every Christmas, the Fairies gather together and lend a portion of their magic to Santa Claus, who uses the magic to transform from a corporate looking man into the jolly, fat, and bearded version most people associate Santa with. He can use the magic loaned to him to create toys and deliver them quickly across the Earth. Once Christmas is over, the magic goes back to the fairies.
One point of interest is in Santa Claus' first appearance, "The Temp!", one of his elves named Jeff escapes from his workshop and poses as a temporary fairy godparent. When he was being return to Santa, Jeff begged Timmy and his fairies to not let Santa take him, while the other elves told Jeff to join them in a droning voice. This is the only time Santa is shown in a slightly villainous light, at least from the point of view of Jeff the Elf. Santa also appears in the next short, "The Zappys!", in a brief cameo as he is seen entering the Zappy award contest on the red carpet along with Cupid and several other magic beings.
In his first major appearance, "Christmas Everyday!", Timmy wished it to be Christmas everyday, so he can spend more time with his parents. This backfires due to the constant magic that Santa keeps absorbing every Christmas eventually causing him to run out of ideas due to him absorbing almost all the magic he got. The other holidays try to get rid of him, but Timmy and all of the other kids in the world stopped them from succeeding. Thanks to the help from the other kids, Timmy is able to unwish his wish returning everything to normal. Da Rules then forbidden any godchild from wishing that Christmas to be on every day to prevent the same incident from happening again. As Santa was overworked because of Timmy's wish, he and his wife were allowed to have the holiday off the following year while Timmy, his godparents, and the other holidays deliver the presents to the kids.
Santa had a major role in "Merry Wishmas", where Timmy's wish for a "Wishmas" replaces Christmas and puts Santa out of work. Santa, his wife, and his elves end up crashing at the Turner's house until Timmy gets things back to how they were before.
His last cartoon appearance was in the episode, "Dad Overboard", when Mr. Turner gets lost and ends up by crashing into Santa's Workshop, which upsets Santa.
Santa's latest major role was in "A Fairly Odd Christmas", in which he'll be played in live-action by Donavon Stinson. When Timmy and Tootie's company, Wishful Thinking, grants wishes for kids around the world, Santa doesn't have much to do. To make it worse, Timmy accidentally injures Santa, giving him amnesia, and making him believe he's the Easter Bunny and other holiday icons. Now Timmy must replace Santa that year, but as he's in the Naughty List, due to the wish granting issue, he must clear his name first.