Nutty is a light green squirrel with one yellow diamond-shaped marking on his head and a large curled-up tail that moves when he's excited. Nutty has an addiction for candy or anything that is sugary. He has a big swirly lollipop, candy cane, small all-day sucker, and a candy apple stuck to his fur, which he even treats as his wardrobe, as evidenced in Chew Said a Mouthful when he's shown dressing in them and even taking great care in straightening his candy cane like a tie, although this changes at times. In his debut episode Nuttin' Wrong with Candy, the candy attached to his fur fell off, meaning Nutty was seen without candy stuck to him for the first time.

It's apparent that his googly eye is an amblyopia, or lazy eye, as it's almost always looking down and drops down whenever he looks somewhere else; it also bounces around when he jitters. In some episodes, however, he seems to be able to control it. Due to animation errors, Nutty's lazy eye changes its position multiple times. For example, in Icy You, his lazy eye was his left eye. After he noticed the slushy machine, it was his right eye for the rest of the episode.

As a result of his huge sugar intake, he is often hyperactive and jittery, and giggles more than he talks (sometimes he even giggles while he's dying). He was first heard actually speaking when he wishes for a scrumptious lollipop in As You Wish. One drop of sugar makes him hyper to the point that he could create a tornado like "Taz the Tasmanian Devil", as seen in Party Animal. He sometimes foams at the mouth when deprived of candy or any other sugar-related items. As seen in the False Alarm episode, he can go insane if left without candy for too long. It also shows that once he loses one addiction, he is susceptible to start a new one almost immediately, hinting he probably has an addictive personality. He also becomes momentarily sad after finishing his candy.
Nutty will do whatever it takes to get the sugar he wants. If he doesn't get what he wants he becomes very irate. Nutty will eat other types of "food", occasionally mistaking them for candy (like in Class Act, when he eats part of Sniffles' body and some cardboard, thinking it was a candy cane, or in Concrete Solution when he eats a whole bag of cement mix, mistaking it for a bag of sugar), but this is very rare. He will also eat regular food, like pancakes, but he usually drenches them in syrup first as seen in Concrete Solution.
He is one of few characters to have an account on the website MySpace, where it mentions he has fourteen dental cavities. This is probably not helped by the fact that he brushes his teeth with a lollipop and sugar, and uses soda as mouthwash. He drinks coffee ground from jellybeans and still has to add more than a bowlful of sugar to the beverage to make it sweet enough for him. Given this information, it's actually surprising that he doesn't have more.
In the Second Serving DVD, he has his own breakfast cereal called "Sugar Frosted Nutty Honey", but there are hardly any cereal pieces in it - the picture only shows a large pile of sugar in the bowl. Nutty can be seen eating this kind of cereal in Concrete Solution.

Nutty is usually featured in episodes revolving around candy, except for Eye Candy, where Toothy was used instead and the pink and yellow candy Petunia is shown holding in the front cover of the Volume 1 DVD. It was rumored that when Eye Candy was made, Nutty had already been prominently featured in another episode, which was why that choice was made.
Nutty's love for candy has often led to the demise of both himself and others, but it is his hyperactive personality that has made him one of the show's most memorable characters. Nutty cares more about candy than he does about his friends. In Party Animal, he pushes aside an allergy-stricken Flaky to eat chocolate. In A Sucker for Love, he tried to steal a lollipop from Cub. In Chew Said a Mouthful, he stole a gurney with a badly injured Toothy on it so he could get a jawbreaker. In Concrete Solution, he stole a bag of sugar from the Happy Tree Café, and he stalked Cuddles for his ice cream in Sweet Ride. He seems to lack common sense (which is evidenced in many episodes), especially when sugar or what he believes to be sugar involved, something that commonly leads to his death. His deaths usually involve his mouth, getting impaled, getting crushed, split apart, shredded, or breathing problems, and some of those deaths Nutty might deserve due to his cruel actions towards other in his love for candy.
The only episodes he survived in are Nuttin' but the Tooth, I Get a Trick Out of You, Out of Sight, Out of Mime (DVD only), Who's to Flame? (debatable), Mime to Five (debatable), A Change of Heart, In a Jam, Milk Pong, Happy Tree Friends: False Alarm Episode, Camp Pokeneyeout, Something Fishy, the HTF Break shorts Bite Sized, the YouTube Live Episode (debatable), and Claw (debatable).