Kindhearted Simpleton

There are many characters who are dumb or airheaded. However, for some of these, what they lack in intelligence they make up for in compassion and altruism. The Kindhearted Simpleton isn't the sharpest tool in the shed, but is always well-meaning and compassionate to other characters. Probably will not be quite as dumb as a standard Ditz or as airheaded as a Cloudcuckoolander, since that is not the focus of their character.

Due to their well-meaning nature but below-average intelligence, these characters have the potential to be taken advantage of and manipulated by smarter (and occasionally evil) characters. The Kindhearted Simpleton should beware of Manipulative Bastards and Guile Heroes. (They should also know it's a snake when they bring it in.)

Contrast Insufferable Genius and Mad Scientist. May be a Gentle Giant. When they are lucky and carefree instead of compassionate, they're The Fool. See also Idiot Hero, Dumb Is Good and Emotions vs. Stoicism (this landing on the "emotions good, logic bad" side).

Examples of Kindhearted Simpleton include:

Anime and Manga

Prince Odysseus, the formal heir apparent to the Britannian Empire in Code Geass, is this, in stark contrast to his father and most younger brothers (and sisters, too). It says a LOT that a guy like him is the "sanest" among the horribly fucked up royal family
In Suite Pretty Cure, Hummy is a rather dense optimist who is unable to hold grudges or remember depressing events for long, allowing her former friend, Siren to take advantage of her. However, these same traits eventually help Hummy win Siren back over to the side of good.
The title character of Cardcaptor Sakura is a borderline example. She's more spacey and incredibly naive than outright stupid (and shows incredible tact and skill when the time requires it), however it is obvious her heart is much bigger than her brain. Then again, she is just 11-going-on-12.
Keith Goodman/Sky High from Tiger and Bunny is one of the nicest guys you could meet. He is also an unquestionable airhead.
The titular Sailor Moon.
Ash Ketchum from Pokémon.
Tsubasa from Captain Tsubasa.

Film

Karen, the Brainless Beauty Minion with an F In Evil Amanda Seyfried played in Mean Girls is technically part of a bitchy clique, but she doesn't come across as much of a bitch herself.

Live Action Television

Kenneth from 30 Rock.
- Although there are multiple blink-and-you-might-miss-them moments that imply that Kenneth's life might be a bit Darker and Edgier than it seems on the surface. One example that comes to mind is when he rattles off one of his family's "classic Southern recipes." One of the ingredients he lists is a Union soldier.

Literature

Inch from the Memory, Sorrow, and Thorn trilogy is a serious subversion. He's simpleminded, but EXTREMELY cruel and sadistic. And possibly a pedophile.

Comic Books

Taipu from Bionicle, to the point where his friend Onepu constantly takes advantage of him.
In Usagi Yojimbo, a mentally challenged man is a great friend of Kitsune. Unfortunately, he's framed for one of her crimes, and is executed for it, despite him being one of the nicest people in town. Kitsune (and other people outside the comic) was driven to tears.

Video Games

The titular character in MARDEK, who is also an Idiot Hero.
Sonic the Hedgehog: Big the Cat is slow witted and even has the voice to match, but he is one of the most mellow and soft hearted of the protagonists. Don't be mean to little frogs or innocent bunny girls around him.
Crash Bandicoot, while slightly more sociopathic than usual examples, often leans into this in certain titles. He was in fact rejected as a minion by Dr Neo Cortex for being too nice.

Web Original

Son Goku of Dragon Ball Abridged, with him having Took a Level in Jerkass in the first two unofficial specials and the first two eps of the main series. He later Took a Level In Kindness after his death following the second ep.

Western Animation

The titular character of SpongeBob SquarePants. While he is responsible for most of the havoc in each episode, he usually means well, and is something of an Extreme Doormat.
Winnie the Pooh is a gluttonous idiot, but extremely gentle and friendly. Most of the supporting cast aren't far off this trope either. This is prominent in the original novels too, though most prevalent in the Disney adaptations.
The latter half of Ren and Stimpy in contrast to the smarter but manic and violent tempered Ren.
Rufus of The Dreamstone is a borderline example at times.
Butters Stotch, with his occasionally tragic innocence and positive outlook stands out like a sore thumb in the Crapsack World of South Park.
Mikey Blumberg from Recess is The Messiah and the Cloudcuckoolander of the series.
- To a certain degree, same with T.J. Detweiler. He's the living eptomime of Book Dumb but he's also a sweetie and means well towards everyone.
Niblet fills a similar role to Mr. Blumberg on Paul & Joe's version of Pound Puppies. He's as kind and sweet as can be, but he can be mind-blowingly dense at times.
Ed is Made of Iron, has Super Strength and can warp reality through his own idiocy. He also happens to be the absolute nicest character on the show, without a mean bone in his body... Unless he has a pebble in his shoe... Or you hurt his best friends.