He is the younger son of the overprotective Mama, who did not approve of his marrying Wanda in the least bit and has often tried to separate the two. He was once the last Fairy ever born, as his birth caused the ban of fairy babies, until Poof came. He apparently also was in a band where he played the electric triangle, though tomatoes were thrown at him every time he has played.
Cosmo is usually the cause behind Timmy's crazy adventures, often misunderstanding circumstances, or greeing to grant a dumb wish. He will sometimes cause Timmy to realize what has to be done to reverse a wish, but this does not happen often. He keeps a diary, which has all of his deepest, darkest secrets, including the fact that he is afraid of the hunt. He is often the subject of magic from Wanda, who usually does something to him to shut him up, including zipping his mouth, and then encasing him in vanadium.
There are countless other scenarios. He is often rather careless, but loves Wanda and Timmy Turner through his destructiveness. Cosmo is noted to be one of the most powerful fairies ever, seeing as he blew up all of Fairy World in the first 5 seconds after he was born, sunk Atlantis 9 times, destroyed Fairy World again 5 times, caused Mt. Vesuvius to erupt and destroy Pompeii, transformed Xanadu into Pittsburg, summoned up a gigantic three-headed hydra, whom he dubbed "Snowball", with one careless wave of his wand, and was able to accidentally wish away Wanda's wand and make wands in Wishology, which is something that it is said that fairies cannot do when they say that magic cannot wish away magic.
Cosmo also has had a big crush on the Tooth Fairy in some episodes but yet never had a chance to show it because she is in a relationship with Jorgen. Cosmo is also very ticklish.
Ever since the day Cosmo was born, he has been causing chaos. When he got a magical rattle, he accidentally used it to turn Jorgen into a bomb that blew up a portion of Fairy World. The second he got old enough to receive his first wand, he turned his father into a fly, never to be seen again. Later on, because of his dangerous lack of control over his magic, his Mother sent him to the Fairy Godparent Academy, where Jorgan taught him as well as many others. Cosmo has then been known as the worst thing to happen to Fairy World everywhere. Examples include: when fairies learned accuracy with their magic and turned flying plates into birds and butterflies, he turned his plate into a destructive hydra that he named "Snowball", which became his new pet, and while fairies discovered the strange place of Atlantis, Cosmo sunk it 9 times, gaining the nickname "The Accursed One". This caused Jorgen to strongly despise Cosmo.