Guardian Angel

“See that you do not despise one of these little ones, for I say to you that their angels in heaven always look upon the face of my heavenly Father.

—Mark 18:10

A Guardian Angel is a benevolent supernatural entity, whose main function is to protect and guide a person.

The concept itself seems to originate in The Bible. In the Old Testament angels are described as entities that carry out God's will, which may include helping people in their personal affairs. New Testament makes a reference to personal angels. Over ages the concept evolved more.

Like cherubs' typical description evolved over time from Eldritch Abomination to a cute baby-like being, guardian angels became the "good angel" in a good angel-bad angel routine. The bad angels, too, originated from the same concept, as it was commonly believed, though non-canon, that there is a fallen angel following around people as well, as the fallen angels seem to attempt to recreate the celestial hierarchy.

Guardian angels, generally speaking, do not have as much power, they typically work in subtle ways and often act as a conscience. Sometimes they are even reduced to mere scribes, recording the person's good deeds, then there will inevitably be an imp that records evil deeds, so that it can be determined if a person's soul goes to heaven or hell (that's the canon in Islam).

Zoroastrism has similar entities called Holy Guardian Angels. They are more or less exactly like Christian Guardian angels, but interestingly they chose to help people themselves, as opposed to Christian Angels, which are God's servants.

In fiction, the role of guardian angel is sometimes held by a dead person, who may or may not have known the person they're guarding when they were alive, instead of by an actual angel.

Expect God Was My Co-Pilot, as usually the person is not even aware that the guardian angel influenced anything or is even there. Expect Angel Unaware if he shows up in person. May also be a Spirit Advisor.

Compare Guardian Entity, Fairy Godmother.

Examples of Guardian Angel include:

Fan Works

In the Harry Potter/Sailor Moon/Ranma ½ crossover The Girl Who Loved, Luna Lovegood claims to be able to see (and communicate in a limited way) with them. She claims that Harry's has taken the name "Luke", for Luke Skywalker, and she's allegedly alerted to an attempt to dose Hermione with a love potion by the state of her Guardian Angel. Given what we learn of Luna's abilities in the story, there is a non-zero chance that this is literally true, and not a bizarre metaphor or part of her "Looney" act.

Film

Clarence, the angel from It's a Wonderful Life.
The angels of Angels in the Outfield are a somewhat different version than the standard Guardian Angel but they are examples of them.
One appears at the end of What Women Want

Literature

Sookie's Fairy Godmother was studying up to be promoted to Guardian Angel in The Sookie Stackhouse Mysteries.
In Philip K. Dick's Eye In The Sky, at one point the protagonists end up in a world (actually a fundamentalist's personal mental world) where religious concepts are physically manifest and obvious. One of these are guardian angels, who can manifest as small mouths that whisper advice into a person's ears.
In Gene Stratton Porter's Freckles, after Freckles has let slip that he thinks she's an angel, he gives her food and she argues that he's her guardian angel.

Live-Action TV

Charmed:
- A specific episode in season seven features guardian angels, named as such in series.
- More generally the "white-lighters" fill a very similar role, although they are deceased humans, not angels. The sisters' white-lighter watches over and helps them as a force of good and connection to the Big Good in the sky (i.e. God).
Touched By an Angel sort of does this, as does Highway to Heaven.
Quantum Leap episode "It's a Wonderful Leap:" a woman claims she is Sam's guardian angel, and is there to help him help the person he leapt into. She can see Al, who is Invisible to Normals.
The plot of Teen Angel: Marty dies, but hangs around his best bud Steve and helps him.
Some of the angels in Supernatural were supposed to be this.
The Latinoamerican Telenovela Un Angel Llamado Azul (An Angel Called Blue) was about a little girl's guardian angel who falls in love with her teacher.
The short-lived 1979 series Out of the Blue revolved around Random, a trainee angel assigned to live with and serve as the guardian angel to a suburban Chicago family while maintaining a cover identity as a high school teacher.

Newspaper Comics

A recurring theme in The Family Circus.
Pasquale's is a main character of Rose Is Rose

Tabletop Games

In In Nomine, the Cherubim are the choir of Guardian Angels. They attune themselves to the person or object they're watching over, allowing them to always know where it is and its general state.

Theatre

In the opera Hansel and Gretel, the children's evening prayer sings of fourteen angels watching over them as they sleep. These fourteen appear in the ensuing Dream Ballet.

Video Games

The Pokémon Gardevoir is partly based on the guardian angel - its Pokedex entry states that a Gardevoir will guard their Trainer with their life, and the name is a portmanteau of the French words for "guard" and "duty". Its Japanese name, Sirknight, also evokes a similar image.
Yumei-san in the Girls Love Visual Novel Akai Ito, of the dead-person variety. Saying that she's angelic would be a massive understatement. Look for Incorruptible Pure Pureness in the Akai Ito page for details.
NetHack makes it possible to encounter one if you reach the Astral Plane with no active conflict effects and a good alignment record.

Web Comics

In Sorcery 101, Sorcerers who are demon-hunters have angels who watch over them and guide them but aren't allowed to interfere much. Cal in particular was Ally and her family's angel.
In PS238 having one of these is Guardian Angel's power. It does things such as prevent security systems from picking her up, stopping a sentient cloud of nanites from possessing her, protecting her from the rain, preventing anyone from delivering life saving treatment to her after it was temporarily shut off and preventing her moving on to the afterlife until she was resurrected by a a classmate with healing powers.
In Misfile, Cassiel has decided to assume this role towards Eponine, trouble is Cassiel is not that competent about it.
In El Goonish Shive, Nanase gets a spell with this as its name which transforms her into the form of an angel and gives her the abilities needed to protect Ellen from Abraham.

Western Animation

Daria has an episode where Quinn was convinced she had one.
Melody Time: Johnny Appleseed has a guardian angel resembling a bearded old man wearing deerskin clothing and a raccoon hat.
Heathcliff and The Catillac Cats: Heathcliff had a guardian angel that, in one episode, kept hitting him with the halo.
Kenny from South Park, as his alter ego Mysterion, acted as a guardian angel to his younger sister Karen in the episode The Poor Kid.
Johnny Bravo had one in "Johnny's Guardian Angel".
Rugrats: Chuckie had one in an episode parodying It's a Wonderful Life (except replace contemplating suicide with contemplating running away.)