10 - Paloma & Wanda Pepper - Both aliases used by Elaine, Paloma is the name she used when she posed as Uncle Leo's nurse in order to get a diagnosis for herself (she is a difficult patient). Wanda Pepper was an alias she used in order to retrieve a jacket for Kramer.
9 - Dylan Murphy - Jerry's made up name when he and George take Donald O'Brien's limo. They think they are going to watch the Knicks play, but soon realize they are actually on their way to a rally where O'Brien, the head of the regional chapter of a Neo-Nazi organization, is supposed to give his first public speech. Oops.
8 - Eduardo Corrochio - When George and Elaine go to dinner with a married couple, Elaine tells everyone this elaborate story that she once dated a romantic Spanish matador, whose name she makes up on the spot. What a great name!
7 - Kel Varnsen - The alias used by Jerry in order to help George with Vandelay Inc. He uses the alias again when he joins Kramer (posing as a wealthy industrialist) to watch the Mets game in an apartment that is up for sale.
6 - Whitey Fisk - George's imaginary summer friend. George was always jealous of Jerry and his summer camp friend, Fragile Frankie Merman. Whitey Fisk was supposed to be his "summer Jerry", but it was all a farce.
5 - Martin van / Peter von Nostrand - One of Kramer's favorite alter egos, sometimes he is Dr. Martin van Nostrand, once in order to get Elaine's medical chart. He also posed as Martin to audition for the role of Kramer in Jerry & George's pilot, Jerry. Kramer also presented himself as Peter von Nostrand in an attempt to retrieve a jacket from a man's apartment.
4 - Snoopy & Prickly Pete - The names of George's two imaginary horses he keeps at his fictional Hampton's estate. "All right, we're taking it up a notch!"
3 - H.E. Pennypacker - Kramer posed as a "wealthy industrialist" a couple of times - most memorably in Putumyo clothing store to help Elaine get revenge and again when Kramer needs to use the bathroom and sees an apartment for sale.
2 - Susie - Elaine's alter ego, Susie is believed to be real by at least two of her co-workers. When Elaine tries to kill her off, a funeral is arranged for her and Peterman claims to have had an affair with Suze (she hates when people call her that).
1 - Art Vandelay - George's alter ego is in so many episodes it is hard to count or keep track. Sometimes George works for Vandelay, sometimes George IS Vandelay, and in the Finale, the judge is actually named, Arthur Vandelay.