"... the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light ..."
-- Catullus Spangle, 18th century Charms researcher in his masterwork Charms of Defence and Deterrence (Pm)
A Patronus is a silvery-white image of a creature created by using the Patronus Charm. Lupin describes a Patronus as “a kind of positive force, a projection of the very things that the dementor feeds upon — hope, happiness, the desire to survive — but it cannot feel despair, as real humans can, so the dementors can’t hurt it.” Catullus Spangle, an 18th century Charms researcher, wrote in his masterwork Charms of Defence and Deterrence that “the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light …” (Pm).
A Patronus can be commanded by the caster to attack. When Harry and Dudley were attacked by two Dementors in the alley near Privet Drive, Harry controlled his and told it to attack each Dementor in turn (OP1).
A Patronus can also appear as a shapeless silvery mist, but when cast correctly, it forms a “corporeal Patronus,” which means it takes the form of an actual creature (corporeal means having a physical form, from “corpus” L. for body). (OP8)
After a severe emotional upheaval, a witch’s or wizard’s Patronus may change form (HBP16); when Harry saw Tonks produce a wolf-like creature, he thought she was upset over the death of Sirius, a dog Animagus. In truth, her Patronus had changed to match that of Remus Lupin, and became a wolf (HBP8, Pm). Originally, Tonks’ Patronus was a jackrabbit (Pm).
A Patronus may shield against other Patronuses (BoS).
References from the canon
- Harry's Patronus is a stag (like his father's Animagus form) (PA21, OP1).
- Andros the Invincible is alleged to have been the only wizard known to have produced a Patronus the size of a giant (FW)
- Hedley Fleetwood had a rare extinct Patronus of a woolly mammoth, while Symposia Rawle had a tiny ladybug (BoS)
- A young wizard named Illyrius protected his town from an "army of Dementors" with his small mouse Patronus (Pm)
- Aberforth Dumbledore's Patronus is a goat (DH28).
- Albus Dumbledore's Patronus is a phoenix (DH20, JKR)
- Hermione's Patronus is an otter and Cho's is a swan (OP27). Seamus at first wasn't sure what his was, just that it was hairy; it turned out to be a fox (OP27, DH32).
- Luna Lovegood's Patronus is a hare (DH32).
- Ernie Macmillan's Patronus is a boar (DH32).
- Kingsley Shacklebolt's Patronus is a lynx (DH8).
- Severus Snape's Patronus is a doe (DH33).
- Dolores Umbridge's Patronus is a cat (see) (DH13).
- Ron's Patronus is a Jack Russell terrier (JKR, DH32).
- Arthur Weasley's Patronus is a weasel (DH7, DH9).
- Minerva McGonagall's Patronus was a cat with markings around the eyes similar to her Animagus form (DH30).
- Flavius Belby survived a Lethifold attack by casting a Patronus Charm against it (FW, FB)
"patronus" Mediaeval L. patron saint
There are still some mysteries concerning the Patronus Charm which have not been satisfactorily explained.
The author said that Death Eaters have no need of a Patronus Charm:
Rowling: Yes, because a Patronus is used against things that the Death Eaters generally generate, or fight alongside. They would not need Patronuses (BLC).
However, sadistic Dolores Umbridge of the Ministry was able to do the charm, creating her cat Patronus to protect her from Dementors in spite of an evil nature and sad family situation as a child (DH13, Pm). True, she was never technically a Death Eater, but she did wear the Slytherin horcrux locket, which later kept Harry from fighting off dementors with his own Patronus (DH15).
So how was her Patronus at the Muggleborn Registration Commission possible? The author explained that Umbridge was "a very nasty piece of work. She has an affinity for this horrible object, which would help rather than hinder her" (BLC). This would still seem to be an inconsistency, since Umbridge had no way of knowing the locket was a dark object, and merely wore it for prestige. Perhaps Rowling is suggesting that the bit of Voldemort inside the horcrux felt an affinity for Dolores, so allowed her a Patronus for protection, but that is just speculation. However, a Patronus is described as a "soul guardian" which is the opposite of a torn-soul horcrux (JKR), so it is hard to see how the two things could ever work together to create a creature of light.
Perhaps the best explanation for why Umbridge could create a Patronus lies in her attitude, that a "true and confident belief in the rightness of one’s actions can supply the necessary happiness" (Pm-Patronus Charm)." In other words, although Dolores was a dark witch with a horrible life, she believed in herself and that made her happy enough to master the Patronus Charm.
But there was one Death Eater who reformed and tried to do the right thing who did indeed have a Patronus: Severus Snape, who was able to conjure a spirit guardian thanks to his happy memories of his childhood friend, Lily Potter. After watching Snape's memories in the Pensieve, Harry told Voldemort that the proof of Snape's love for his mother Lily rested in their shared silver doe Patronus.
“Snape’s Patronus was a doe ... the same as my mother’s, because he loved her for nearly all of his life, from the time when they were children (DH36).
This statement is somewhat of a mystery because nowhere in the books were readers ever told about or shown the Patronus of Lily as either a child or a grown-up. So while Harry's statement is canon, it is hard to say how he reached the conclusion that Snape's Patronus must be identical to Lily's, since Harry had apparently never seen Lily's Patronus.
It could be that Harry made a leap of deduction since his father's Animagus form as a stag influenced his own stag Patronus, then perhaps he assumed that his mother's would have the form of a female deer, just like Snape's. The author seemed to hint at that possibility when she answered a chat question this way:
Question: James Patronus is a stag and Lilys a doe is that a coincidence?
J.K. Rowling: No, the Patronus often mutates to take the image of the love of one's life (because they so often become the 'happy thought' that generates a Patronus) (BLC).
The person asking that question assumed that James's Animagus form was also his stag Patronus form and the author did not contradict that. But to be clear about the canon, the Patronus of James is never mentioned anywhere in the books. And did James create a Patronus before or after seeing Lily's, or did hers reflect his? The only fact we have is that it is possible for an Animagus to have the same animal form and corporeal Patronus, as with Minerva McGonagall, a cat Animagus with a cat Patronus (DH31). And then there's Remus Lupin who transforms into a werewolf each month, and his Patronus is also a wolf.
But to get back to Snape: when did he ever see Lily's Patronus? The easiest explanation would be that he and Lily practiced the Patronus Charm together as children, perhaps in preparation for the O.W.L. exams. It's possible because Lily's willow wand was said by Mr. Ollivander to be "nice for charm work" (PS5). But Patronus practice is never shown in Snape's memories (OP26,DH33), and the Patronus Charm is advanced magic not taught at Hogwarts (Pm). That is why Draco had no Patronus, and Hagrid was unable to master the complex charm (JKR,JKR:Tw). So we still don't know (and may never know) when Snape first learned the Patronus Charm, whether Snape had an original Patronus as a child which changed over time to the silver doe, or whether his Patronus had always matched Lily's.