RPK is Upotte's personification of the RPK light support weapon. She's tall, busty, athletic, extremely competitive, and a staunch supporter of Red Steel High's ideology.

She normally wears the same standard Sailor Suit uniform as her classmates, but stands out due to her distinctive red lipstick. She's also been depicted wearing a maid outfit, overalls and talk top, and she uses a Fox mask when she wishes to hide her identity.

In the Upotte and /ak/ communities, she is often depicted as a futanari and many running jokes about her "extra parts" exist. Hence the "RPK has benis" meme. This seems to have originated from a doujin made by Cosine, but it's possible Cosine was just working off of an earlier joke that wasn't present in the English-speaking part of the community.
rpk (upotte!!)