Place it right at the reveal, just as the character appears, then follow it with the Shocked Meme sound for the reaction shot. For a Japanese-style exchange, open with Nani Sore before it, so the confusion builds into the big entrance.
About this sound
A single voice stretches one long “yoooo” in a theatrical Japanese-style delivery, holding the note for just over two and a half seconds before it ends. The effect is cheeky and overblown, making a sudden appearance, a shifty glance or a swaggering pose feel instantly more dramatic, awkward and ridiculous.