William "Willy" Gilligan is one of the main characters. He is a close friend of the Skipper, and first mate aboard the S.S. Minnow. Despite his accident prone nature, the Skipper often addresses Gilligan as "little buddy", demonstrating their friendship. He served with the Skipper in World War II and apparently saved his life by pushing him out of the way of a depth charge, but according to the Skipper he "didn't exactly save it [his life], You just prolonged it so you'd give me more bad luck." Gilligan was played by actor Bob Denver.
Gilligan has to be the most dimensional character on the island. While much of his past is a mystery, more has been discovered about his personality than any of the other castaways. His personality can be labeled as funny, sensitive, klutzy, dumb in a funny way, and scared, due to the fact that throughout the series he is always doing clumsy and funny things, but he is nonetheless thoughtful and kind. Usually, when faced with headhunters and kidnappers, Gilligan is visibly scared, though when things get serious, he has been known to display a brave streak. Obviously not very intellectual, Gilligan sometimes doesn't understand words like "navigable" or "gourmet" and is very forgetful. Gilligan is accident-prone, and usually, disaster follows him wherever he goes. In Court-Martial, when the Skipper is trying to reconstruct the accident that marooned the castaways, it is discovered that Gilligan lost the anchor which wasn't tied to the Minnow and accidentally tossed the blowtorch overboard.
Gilligan is a prime example of "arrested youth syndrome," otherwise known as a "Peter Pan Complex." In many ways, he thinks of himself as a young boy, enjoying playing, goofing off, making silly comments and reading comic books, but despite all this, he works very hard to please the Skipper and is eager to make him happy. There are several indications he was a bad student in school; he often has a hard time remembering details and keeping his mind on track. It is not truly known why he holds on to his youth, as all of his childhood stories from back home reflect a very happy "Our Gang"-like past, especially with friends named Stinky Sullivan and Skinny Mulligan. This is reflected in his playful personality; even when he gets hurt, he says he sees stars, "but they're shaped like hot dogs." Because he thinks of himself as a young boy, Gilligan often gets nervous around the girls, especially if they try and kiss him, especially Ginger and Mary Ann, who both seem to have a crush on him at times. Gilligan is also vocal about not getting married, even in a pretend marriage to Mary Ann.
Gilligan also seems conscious that he is a screw-up. In his dreams, he often finds himself losing swords in sword fights, getting caught by giants or accidentally shooting his pet duck. Especially sensitive, he has left the main camp on the island to live in the caves on the other side of the island several times, but the others are so protective of him that they always bring him back. For some reason, they often accuse him of lying or making up stories, but it is very obvious that Gilligan is not really much of a prevaricator. His crazy stories are often the product of his hysterical or confused efforts to describe what he has seen. He has confused a one-man submarine as a sea serpent, seen a head-hunter on the island, spies disguised as a ghost and himself and a few visitors to the island before the others, such as Dr. Boris Balinkoff and Lord Beasley Waterford, before the others, but he is almost always correct if not totally accurate.
Gilligan has a close friendship with the Skipper and sees him as his best friend, role model and mentor rather than his employer. Together, they have developed a rather "Laurel and Hardy" relationship evident by the Skipper often hitting him with his cap, worrying about his welfare and sometimes looking exasperatedly out to an (imaginary) audience. Like Stan Laurel, his comedic counterpart, Gilligan has had moments of "white magic," sometimes creating incidents that defy explanation, like flying with wings, levitating a table and redirecting Voodoo back on the Witch Doctor. This would further explain his temporary feats of super-strength in Pass the Vegetables, Please, magnetism and invisibility in Gilligan's Personal Magnetism and ability as a radio in Hi-Fi Gilligan as well as ability to elude Jonathan Kincaid in The Hunter.
Gilligan always wears a red long-sleeved polo shirt with a white collar, blue bell-bottom jeans, both white socks and sneakers, and a white trademark sailor hat. In Smile, You're on Mars Camera, it is said that his red shirt originally belonged to his brother. Although blown up beyond repair in Bang! Bang! Bang!, Mary Ann somehow restores them to immaculate condition. However, in Ghost a Go-Go, it's suggested that he has multiple copies of the same shirt.
Gilligan brought with him to the island several things he probably kept on the S.S. Minnow, including comic books (Fossil Face, Davie Davis, Buck Rogers...), his diary, a lucky four-leaf clover lucky charm, a pocket knife, baseball cards, a deck of cards, his library card, a two-year-old Argentinian lottery ticket (#G131131), his bank book, a calendar, two marbles, seven cents, a secret compartment spy ring, a platinum yo-yo, Yogi Bear bottle tops, wheels from an old skateboard, a Manny Moose wristwatch, a boy scout handbook and his high school graduation ring from Girl's High. (Some mothers keep their kids in curls way too long.) He tries leaving some of his stuff to the others in Hair Today, Gone Tomorrow, but he's mostly just leaving everything around the island. He also finds four pearls and an onyx the Skipper believes is a wish-granting Eye of the Idol.
- Sherwood Schwartz has always answered that he intended to reveal Gilligan's first name as "Willy," but Bob Denver always insisted it was just "Gilligan." In fact, in the first episode, he is referred to as "a first mate known only as Gilligan."
- Gilligan's birthdate is never given, but he was born in "a tiny town in Pennsylvania" on a stormy night. He is in his 30s during the show. However, in Meet the Meteor, if Gilligan had aged 50 years, he would have been 72; thus, his age in 1966 would have been 22, suggesting he joined at Navy at 18 when he met the Skipper.
- He has a mole in a very private spot.
- Gilligan mentions quite a few relatives in the series, describing his father as short with a mustache and glasses, later adding that he gave his father hives. He has a brother whose shirt he has been wearing during the series. It turns out he was reluctant to appear on camera in Smile, You're on Mars Camera because he is afraid that his brother will punch him for wearing his shirt!. He also mentions his mother, an Aunt Martha with a rutabaga recipe, his Aunt Sara who had a "sneaky disease," his grandfather Everett, an Uncle John who is likely deceased and can't read, his Cousin Rudolph, who he went to the movies with him at ten-years-old with his friend Claude, his Uncle Ramsey who was a guide for the "Lost Battalion."
- When he was three-years-old, he left his pail and shovel at the beach. Gilligan recalls in Forward March that he cried over it.
- Gilligan mentions several friends with "Our Gang"-like nicknames from his home town including Stinky Sullivan, Skinny Mulligan, and Bobby McGuire.
- His barber back home is named Sam.
- Before being on the island, Gilligan was once president of the eighth grade camera club; he was the only one who owned a camera.
- He graduated from a girl's high school. In Physical Fatness, it's said that he was expelled from his old school when he nearly started a chemical fire.
- In the second episode, the Skipper alludes to the fact that he met Gilligan while they were both in the Navy.
- Gilligan mentions in Birds Gotta Fly, Fish Gotta Talk that he's afraid of heights.
- His chores on the island include collecting freshwater and fire wood. He also covered breakfast until Mary Ann took it over. He also covered the laundry for a while and played a steward for the Howell Country Club on the island.
- One of Gilligan's favorite dishes on the island is Mary Ann's coconut crème pie.
- Gilligan is part of several "recurring gags" in the series. When he gets scared, he always runs into a straight line into the forest at lightning speed, such as in The Invasion and Slave Girl. Another gag is that whenever Ginger tries to kiss him, he always knocks himself out on something. Another gag is that he always knocks down Mary Ann's drying laundry on the clothes line, even as she hurries to take it down. He is also known for crashing into things when he swings on a vine and coming down on top of the Skipper from out of palm trees, even when the Skipper has moved out of the way.
- In his dream sequences, Gilligan still never does anything right. In The Sound of Quacking, Gilligan as a Town Marshall saves a duck from the rest of the castaways, only to shoot it by accident. In The Sweepstakes, Gilligan is a town marshal who shot Mary Ann's grandmother. In Court-Martial, while he's throwing swords to the pirates so it'll be a fair fight, he accidentally throws his own sword. In Lovey's Secret Admirer, Mrs. Howell sees him as a well-meaning but inept fairy godfather who turns her as Cinderella into a donkey. In Up At Bat, when Gilligan thinks he is a vampire, he tries to fly down on his victims from an alcove, except he forgets that he doesn't have an alcove!
- Gilligan is referred to as a Lieutenant in Episode 13 of Gilligan's Planet.