He is a con man, and tricked Cosmo into giving him his wand, a jar of pickles, and a TV, to find an evil scientist who does not really exist as it was just a lie. But as gullible as Cosmo is, Schnozmo thought he'd never find out, but when Wanda followed him and Cosmo heard he was lying, his heart broke, and Schnozmo felt so sad for his little brother that he pretended to be a spy on a play made by Wanda, getting mauled along the way, just to make Cosmo believe in him again.
Schnozmo looks almost identical to Cosmo, sharing the same green colored hair and eyes, but Schnozmo has a much larger nose than Cosmo and all other characters on the show, hence his name. He also wears a tuxedo like a secret agent, although its not known if this is his actual every day attire since he is not really a secret agent.
Not much is known about Schnozmo, because he has only appeared in one episode. He, himself stated that he would never want to hurt his brother, and this is further proven when he takes part in a made-up plot just to spare his brother's feelings. He doesn't get along with his sister-in-law, Wanda, mostly because she doesn't trust him. His relationship with his only nephew, Poof, isn't expanded upon either, but he is surprised to see him, even asking "Who's this little charmer?" only for Cosmo to reply "That's your nephew, Poof!". He also missed Poof's birth, like Wanda's sister Blonda. He has a lot of trouble remembering Timmy's name, calling him Lorenzo, Fernando, and Horatio. It is likely he doesn't get along with Jorgen von Strangle, since he fled from the restaurant he owned, causing Jorgen to go after him. Also it is unknown what his relationship with Mama Cosma is like since we have never seen them together, but it is likely she prefers Cosmo over him. Schnozmo is also older than Cosmo, since Cosmo was the last fairy baby born.
In "The Gland Plan", it was mentioned by Wanda that Cosmo is an only child. This goof might have been caused due to the creators inventing Schnozmo after The Gland Plan. Or it may have been because Wanda wanted to hide Schnozmo's existence from Timmy, since she didn't trust him.