The stag is first seen in
Yet despite the danger of his mission, and despite being hated by most of the wizarding world, Snape held on to the one thing that kept him going: the safety of Lily's son. He even sent his Patronus to guide Harry Potter to the sword of Gryffindor (a known Horcrux-killer) in a nearby lake.
In Deathly Hallows, it is never shown that Snape saved Harry and Sirius during the third year. Snape's patronous is shown to be a doe. Harry's is a stag. In the Deathly Hallows, Snape's patronous is shown to be leading Harry to the sword of Gryffindor in the Dean's forest.
In Potter, Snape's Patronus, a soul-guarding magic spell, takes the form of a deer—more specifically, a doe—as a symbol of his love for Lily, because hers was a doe as well. Now, some of this might just be coincidental, the result of Potter fanatics looking too far into it.
During the attack Harry Potter attempted to defend himself, Hermione, and Sirius with a Patronus Charm, but failed due to the sheer amount of Dementors. However, he and the others were saved by his future self, who had travelled back in time using Hermione's Time-Turner.
The fifth year was boring for Sirius. Except for the fact that he had mastered the Patronus Charm, -his patronus was a black German Shepherd, and became an Animagi, -which was the same as his patronus, the fifth year wasn't very interesting.
The information in this broadcast will prove extremely heartening to Harry, as he learns that he is not alone in fighting Voldemort. It will, however, inadvertently result in the Trio being captured. We learn later that the doe was Severus Snape's Patronus.
Case study 2: Severus Snape
A doe. And in his final battle with Lord Voldemort, Harry explained the significance of this to his adversary, and to us: 'Snape's Patronus was a doe,' said Harry, 'the same as my mother's, because he loved her for nearly all of his life, from the time when they were children'.
With Harry and Hermione despondent after the events in Godric's Hollow and Ron trying desperately to find his way back to them, it was Snape's Patronus – the Silver Doe that was somehow familiar to Harry, perhaps because it recalled his mother's own Patronus – that brought them back together.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Protecting him against Quirrell
During the Quidditch match, Harry believes that Snape is trying to hex him, but in reality, Snape is trying to counteract the hex placed on Harry by Quirrell. It's clear that Snape has been secretly working behind the scenes to help Harry since the very beginning.
Dumbledore reveals how thoroughly he loves Harry. Harry is brave, and Dumbledore is proud as any father would be. More than that, Dumbledore succumbs to a parent's weakness; he cares more about Harry's happiness and wellbeing than the wizarding world he is trying to protect.
The albatross is the rarest Patronus on our list; the one belonging to the lowest number of Wizarding World fans. With the longest wingspan of any bird – up to 11 feet – the albatross surfs the ocean winds for hours, hardly even needing to flap.
So when Ron said three, it wasn't a mistake. He meant there were three more horcruxes to destroy before being able to destroy the fourth and final piece which was in fact Voldemort himself.
In an interview, Rowling described Snape's character as an "antihero". She has said that she drew inspiration for Snape's character from a disliked teacher from her own childhood, and described Snape as a horrible teacher, saying the "worst, shabbiest thing you can do" as a teacher is to bully students.
When asked what shape Draco's Patronus Charm is, Rowling replied that, at least by the end of the sixth book, Draco was not capable of producing a Patronus, as it is not magic routinely taught at Hogwarts.
Snape betrays the Order of the Phoenix. A guy from the Ministry of Magic becomes a Death Eater, which is probably how the Ministry ends up falling.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
He was a double agent, so many character details were kept a secret. His mother was a witch and his father was a Muggle, leading to the moniker "The Half-Blood Prince."
Harry is Lily and James's son, he behaves like his father and his hair is messy like his father's but he has his mother's eyes,thus whenever Snape sees him, he is reminded of Lily and James, but Harry broke a couple of school rules, which reminded him mostly of James, whom he despised instead of Lily, whom he loved.
Draco had several reasons for lying to Bellatrix, including the fact that he was never a bad person. Moreover, he no longer found working for the Dark Lord appealing and hated how Voldemort treated his family. He did not intend to harm anyone and believed Harry was the only person capable of defeating Voldemort.
As the rarest Patronus in the Wizarding World possible, the Albatross represents an enormous bird that flies freely across any sea. This rare Patronus represents those who are simultaneously fearless, optimistic, happy-go-lucky, and ambitious.
Edit: (in case not everyone knows) The rare ones are the following; dragon, unicorn, hippogriff, erumpent, thestral, Abraxen winged horse, fire-dwelling salamander, Granian winged horse, occamy and runespoor.