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 has to be the most dimensional character on the island. While much of his past his a mystery, more has been discovered about his personality that 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 "navigatable" 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 often gets nervous around girls, especially if they try and kiss him (including Ginger and Marry Ann, who both seem to have a crush on him.
Gilligan 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, but the others are so protective of him that they always bring him back.
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.
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, which would make him too young to have been in World War II.
- In the second episode, the Skipper alludes to the fact that he met Gilligan while they were both in the Navy.
- Before being on the island, Gilligan was once president of the eighth grade camera club (he was the only one who owned a camera).
- Gilligan mentions he 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" and his Cousin Rudolph, who he went to the movies with at ten-years-old with his friend Claude.
- Gilligan mentions several friends with "Our Gang"-like nicknames from his home town including Stinky Sullivan, Skinny Mulligan, and Bobby McGuire.
- 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.
- Two running gags are that when the castaways are about to be rescued, Gilligan always fouls things up at the last second and when Gilligan gets scared, he runs in a straight line into the forrest at lightning speed (The Invasion and Slave Girl). Another running gag is whenever Gilligan remarks - "from now on, its smooth sailing...." disaster results. A third gag is that when Ginger tries to kiss him, Gilligan knocks himself out! Another gag is Gilligan always knocks down Mary Ann's drying laundry.
- In in the 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 mistake. 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!
- Ironically, the Old West sets on The Sound of Quacking and The Sweepstakes are actually the "Gunsmoke" TV sets, whose series resulted in Gilligan's Island being canceled before a 4th season could be made!
- A running gag is that whenver Gilligan swings from a vine disaster follows: In X Marks the Spot, he swings from a roof and breaks the Castawys signal mirror; in Gilligan Meets Jungle Boy, he tries to following the Jungle boy on a vine but either falls backwards or lands on the seat of his pants! In the custume party episode Gilligan plays Tarzan, nearly knocks himself out pounding his chest and overturning the castaways' dinner table before crashing into a hut wall... in other words typical Gilligan behavior!