Could a human be a Patronus

Patronus Charm

Incantation

Expecto Patronum[1]
(ex-PEK-toh pa-TRO-num)[2]

Hand movement

Effect

Conjures a spirit guardian[3]

"This ancient and mysterious charm conjures a magical guardian, a projection of all your most positive feelings. The Patronus Charm is difficult, and many witches and wizards are unable to produce a full, corporeal Patronus, a guardian which generally takes the shape of the animal with whom they share the deepest affinity. You may suspect, but you will never truly know what form your Patronus will take until you succeed in conjuring it."
—Miranda Goshawk's overview of the Patronus Charm[src]

The Patronus Charm (Expecto Patronum) is the most famous and one of the most powerful defensive charms known to wizardkind.[4] It is an immensely complicated and an extremely difficult spell, that evokes a partially-tangible positive energy force known as a Patronus (pl. Patronuses[5]) or spirit guardian.[3] It is the primary protection against Dementors and Lethifolds, against which there are no other defence.

There are two types of Patronuses: corporeal which means a Patronus with a particular shape and form and incorporeal Patronus. Incorporeal Patronuses have no particular shape and do not protect against dementors the way corporeal Patronuses do. However, some wizards like Remus Lupin choose to cast incorporeal Patronuses to hide their identity.[1]

History

"And out of the end of his wand burst, not a shapeless cloud of mist, but a blinding, dazzling, silver animal. He screwed up his eyes, trying to see what it was. It looked like a horse. It was galloping silently away from him, across the black surface of the lake. He saw it lower its head and charge at the swarming dementors... "
—Harry Potter's stag Patronus fending off hundreds of Dementors[src]
It is evident from old woodcuts and scrolls that the Patronus Charm has been used since ancient times. Therefore, it is unknown who created it, or when it was first invented.[6] The charm also has a long association with those fighting for lofty or noble causes (those able to produce corporeal Patronuses were often elected to high office within the Wizengamot and Ministry of Magic).[4]

According to legend one of the most famous Patronuses of all time was a lowly mouse, which belonged to a young wizard called Illyius. Illyius cast the Patronus Charm when his village was being attacked by the Dark wizard Raczidian and his army of Dementors. Despite the mouse's diminutive size, it shone with a brilliant light, bringing the Dementors to a halt as it nimbly moved through the ranks of fleeing villagers.[6] Enraged, Raczidian decided to enter the fray himself, and tried to summon a Patronus to ward off Illyius's mouse. However, he failed to remember that only the pure of heart can produce a Patronus, and thus for the first time in history, it was revealed what happens when a competent, but unworthy wizard or witch attempts the spell. Maggots shot out of Raczidian's wand and quickly devoured him as they engulfed his entire body. The villagers hailed Illyius as a hero.[6]

Harry Potter took Anti-Dementor lessons with Remus Lupin in order to learn the advanced charm and so was one of the youngest to attempt this charm with actual results.[1] During a Quidditch match against Ravenclaw in 1994, he cast his very first corporeal Patronus, when several Slytherin students were impersonating Dementors.[7] He performed the charm again in June when he along with Hermione Granger and Sirius Black were being attacked by over a hundred Dementors, he cast a corporeal Patronus powerful enough to drive them all away.[8][9]

In 1995, Harry cast the charm against a Boggart that he thought was a Dementor during the third task of the Triwizard Tournament.[10] The following summer he came under attack by two Dementors and managed to drive them away with the charm, leading to him being put on trial for underage magic in an attempt to discredit him.[11]

Harry also taught the members of Dumbledore's Army the charm and some were even successful in casting it.[11]

The charm was used frequently to ward off Dementors such as when The Trio escaped from the Ministry of Magic after obtaining the Horcrux locket and during the Battle of Hogwarts. Severus Snape also conjured his Doe Patronus to guide Harry to the hiding place of the Sword of Gryffindor.[12]

The Order of the Phoenix would send out messages with their Patronuses such as Kingsley Shacklebolt and Arthur Weasley who in 1997 delivered warnings.[12]

Nature

The Patronus represents that which is hidden, unknown but necessary within the personality. When a human confronted with inhuman evil, such as the Dementor, they must draw upon resources he or she may never have needed, and the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light. Patronuses take forms that their casters might not expect, for which they have never felt a particular affinity, or (in rare cases) even recognise. Every Patronus is as unique as its creator and even identical twins have been known to produce very different Patronuses.[4]

The unusual witches and wizards who produce a Patronus that takes the form of their favourite animal is an indicator of obsession or eccentricity. Here is a wizard who may not be able to hide their essential self in common life, who may, indeed, parade tendencies that others might prefer to conceal. Whatever the form of their Patronus, you would be well-advised to show respect, and occasionally caution, towards a witch or wizard who produces the Patronus of their choice.[4]

Casting

Harry Potter: "And how do you conjure it?"
Remus Lupin: "With an incantation, which will work only if you are concentrating, with all your might, on a single, very happy memory."
— Discussion on how to conjure a Patronus[src]
The vast majority of witches and wizards are unable to produce any form of Patronus, and to create even an intangible one is generally considered a mark of superior magical ability.[4]Rubeus Hagrid is an example of a wizard that cannot conjure any form of Patronus, as the charm is too difficult for him.

Given their long affinity with humans it is perhaps unsurprising that among the most common Patronuses when cast (although it must be remembered that any corporeal Patronus is highly unusual) are dogs, cats, and horses.[4]

To successfully cast the spell, one begins by mustering the happiest memory they can think of (the happier the memory, the better the charm will work).[1] Alternatively, one could imagine a scenario that would make for a very happy memory, as Harry did for his D.A.D.A. O.W.L.[11] The next step is to begin drawing circles with their wand so as to increase the power of their spell.[6] They must then say the incantation, Expecto Patronum; the Patronus will come from the tip of the wand and can be directed towards a target by pointing one's wand at said target.[14] It is possible to disguise the form one's Patronus takes, which was done both by Severus Snape to hide his love for Lily Evans, and by Remus Lupin, who felt it would give away his condition.[4]

Also some witches and wizards may be unable to produce a Patronus at all until they have undergone some kind of psychic shock.[4]

Difficulty

"That's very, very advanced magic."
—The difficulty and complexity of the charm[src]
The Patronus Charm is widely regarded as advanced magic, far beyond N.E.W.T.-level; in 1994 Remus Lupin stated that the charm was, in fact "ridiculously advanced".[1] This charm was, indeed, so perilous, few wizards/witches could conjure up a true Patronus. It is very complex and many qualified wizards and witches have trouble with it.[4] In fact, Harry Potter is one of the youngest known wizards able to cast a Patronus; he was taught how to do so in early 1994 at the age of thirteen by Remus Lupin.[1] In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened.[11] However, three members were later able to cast corporeal Patronuses in the presence of Dementors.[12] Being able to cast a patronus, whilst fighting Dementors or not, is not required to graduate from Hogwarts so it is something that is taught under special circumstances.

Amelia Bones and potential members of the D.A. were all very impressed when Harry Potter revealed that he could cast not only a Patronus Charm but a corporeal one to boot, which is notably more difficult than casting an incorporeal one. Given that the success of the charm is directly reliant on the caster retaining a particular mental state the Charm is more difficult to cast in emotionally trying circumstances. For instance, Harry Potter, Ron Weasley, and Hermione Granger all had difficulty casting their Patronuses when under the negative influence of Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes, and after witnessing the death of Fred Weasley in the final battle.[12] This feature of the Charm is particularly unfortunate since the Charm's primary use is to defend against Dementors, Dark creatures specifically equipped to mentally unhinge people (and hence is partly why it is regarded as such advanced, difficult magic).

Self-confidence may also play an important role in casting the spell. In 1994 Harry is able to cast the spell successfully, driving off a large number of Dementors (which according to Severus Snape is only achievable by a very powerful wizard) thus saving Sirius and a younger version of himself, seemingly without recalling a happy memory, when he realizes that he has already done it, although Harry could have had a 'happy memory' upon realising that he had saved all three of their souls.

Worthiness

It is a general belief held by the wizarding world that only those who are pure of heart are able to cast Patronuses; this, however, is untrue, as several characters with negative personality traits are able to cast a fully-fledged Patronus.[4]