Waver Velvet is a young mage from Fate/Zero, the anime adaptation by ufotable that aired in 2011 and 2012, based on Gen Urobuchi's light novel. As the Master of the Servant Rider, the boastful King of Conquerors, Waver grows from a timid student into one of the franchise's most beloved characters. The '-kun' in this clip is a familiar Japanese honorific, giving the short callout a friendly, teasing tone. Fate fans share the line on soundboards and in memes as a quick way to summon or poke fun at a friend.