Климентина
В допах есть на новом сайте две статьи.
Вот та, что была раньше на поттерморе.
https://www.wizardingworld.com/features/some-interesting-facts-you-may-not-have-known-about-severus-snape
"And Snape was the one who (inadvertently) taught Harry his most-used spell – the one that would end Lord Voldemort
One such imprint is the fact that Snape (albeit unintentionally) introduced Harry to ‘Expelliarmus’, his most-used spell, and the one that would eventually lead to the death of Lord Voldemort."

И вот, поновее.
https://www.wizardingworld.com/features/a-complex-man-exploring-the-light-and-shade-of-severus-snape