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I was surprised, but I think also touched, by the revelation of Severus Snape's childhood friendship with Harry Potter's late mother, Lily Evans Potter.
They were friends long before they attended Hogwarts together, and in fact Snape had felt romantic feelings for Lily almost from the beginning. She, however, always saw him as nothing more than a close friend – that is, until that day when Snape was being picked on by James Potter and Sirius Black, and Lily, as always, defended him and he responded by thoughtlessly calling her a Mudblood in public. Afterwards, she permanently ended their friendship altogether. He tried to reconcile, but sadly never succeeded.
Then, years later, Lily and James married, had Harry, and were then tragically murdered by Lord Voldemort. Snape was devastated by this, and he went to Godric's Hollow the night Lily and James were killed, and Snape cradled Lily's dead body for hours on end, sobbing all the while, as baby Harry screamed for his mother. Snape also felt tremendously guilty for ultimately not being able to make amends with Lily before she died.
From then on, Snape did his best to protect Harry and to aid Albus Dumbledore in bringing down Voldemort. In this way, Snape was dedicated and loyal to Lily, even if she regrettably never knew it. But her son did, and in fact Snape lost his life protecting Harry, something which Harry certainly never forgot. When Harry had his second son, he gave him the middle name Severus.
In the films, Geraldine Somerville plays Lily and Alan Rickman plays Snape as adults. In the last Harry Potter film, they're played as children by Ellie Darcey-Alden and Benedict Clarke, respectively.