His loyalty was constant and unwavering. Some would explain Dumbledore's trust of Snape and Snape's seeming loyalty to him with an Unbreakable Vow between the two.
It's akin to what he himself experiences when he contributes to his sister Ariana's death. For both men, the experience of guilt-ridden grief is a life-altering event. For both men, it is the beginning of a drastic re-examination of their belief systems. Dumbledore trust Snape implicitly because of empathy.
Despite Snape killing Dumbledore, it is learned that they had a special agreement for him to do so. When he died, it was revealed that his deep strong love for Lily Evans caused him to redeem himself, joining Dumbledore's cause for her protection (and, after her death, that of her son) from Lord Voldemort.
The Cursed Ring: Dumbledore was dying from it, and it was painful curse and nothing could be done about it. Dumbledore asked Snape to kill him so that he'd be spared the indignity and pain that were sure to follow.
In return, Snape secretly allied himself with Dumbledore and the Order of the Phoenix as a double agent against Voldemort, using his powers of Occlumency to hide his betrayal from Voldemort.
Why Did Harry Potter Trust Snape in the Deathly Hallows? - Harry Potter Theory
What is Snape's big secret?
This wouldn't be revealed until the final film, Harry Potter and the Deathly Hallows: Part 2, where we learn that when Snape was a young student at Hogwarts, he was in unrequited love with Harry's mom, Lily, and was bullied by Harry's dad, James.
McGonagall eventually hired Neville as the new Herbology professor due to his gifts in the subject, and elected him as the new Head of Gryffindor House. McGonagall later learned that Snape was secretly Harry's protector and had killed Dumbledore at Dumbledore's request.
As a character, Severus Snape was often misunderstood. Many believed he was a villain, and weren't convinced he was actually working for Dumbledore. Snape killed Dumbledore in an effort to protect Draco Malfo and spare him from the emotional trauma of murder.
His father was neglectful and sometimes abusive, which may have contributed to Snape's disdain for Muggles. At some point during his school years, he decided to reject his father's name entirely, giving himself the moniker "The Half-Blood Prince" with his mother's maiden name instead.
Despite the opinions of most others, Albus Dumbledore trusted Snape for reasons that were kept between them both until their deaths. Upon his death, it was revealed that his deep, strong love for Lily Evans caused him to redeem himself, joining Dumbledore's cause at the prospect of her protection from Lord Voldemort.
He hid them using the only thing that would work against Voldemort. He used Fidelius charm and after that even secret keeper was to be protected using that as Sirius said in PoA. The plan failed because wormtail who was their secret keeper betrayed them.
Why Voldemort never discovered Snape's true loyalty?
Because Voldemort thought too highly of himself to think he could ever be deceived. Plus Snape had a way of telling The Dark Lord exactly what he wants to hear.
Until his death, Snape played an impeccable double agent. He was one of Voldemort's most trusted servants towards the end. Voldemort's respect for Snape might mean that by ordering Nagini to be the one who killed Snape, the Dark Wizard was secretly hoping he survived the attack.
Did Snape know Harry was under the invisibility cloak?
Dumbledore was doing James a favour by helping him pass down the Invisibility Cloak to his son Harry. Snape definitely knew because in book 4 when Harry sees crouch in Snapes office, he's in his invisibility cloak. As soon as snape sees the golden egg and the map he says 'potters here' to mad eye. Potters here.
And it really was not for him (Harry) but for her (Lily). And Snape's doe hopped on and away leaving behind Snape filled with pure memory of his love for her. It was then that Dumbledore really understood how much Snape loved her and how real his love was...
Harry Potter had a few love interests during his time at Hogwarts. Though he eventually settled down with Ginny Weasley, he also had a thing for Cho Chang in his earlier school years. Then, of course, there's his secret love for Draco Malfoy… Wait, what?
In the books, Pansy Parkinson has a schoolgirl crush on Draco, and they went together to the Yule Ball. However, there was no true love between them. In the Cursed Child, Draco Malfoy's wife is Astoria Greengrass. Later, after Astoria's death, Draco says that he would “sell [his] soul for another minute with Astoria.”
Luna married fellow naturalist Rolf Scamander, grandson of Fantastic Beasts and Where to Find Them author Newt Scamander, considerably later in life than Harry, Ron, Hermione, and Ginny, who all married and started families in their early-to-mid twenties.
Voldemort told Snape he had been a faithful servant, but that he had to regrettably kill him, as he mistakenly believed he was the master of the Elder Wand, as Snape had killed Albus Dumbledore, its former owner the year prior, during the Battle of the Astronomy Tower.
But what McGonagall doesn't know is that Snape is actually a double agent working for Dumbledore, so he has no intention of harming her. That being said, Snape holds back quite a bit. Many fans, however, like to speculate who would have won the duel had Snape given it his all.
When Harry critically injured Draco Malfoy with the Sectumsempra curse, it was actually enough to have him expelled, as Professor Minerva McGonagall stated Harry was lucky not to have been, due to Snape giving lenience to prevent others from finding out that he was the one who invented the curse.
If at full capacity, defending everything she held dear, the new Headmistress McGonagall was unable to defeat Snape. It suggested the latter was the more powerful professor.
The once shy and awkward Neville Longbottom showed his true bravery during the battle of Hogwarts and eventually became the head of Gryffindor House while also teaching new students Herbology, his favorite subject.
