Any member of one's chosen gender that makes it into that piece of furniture ordinarily used for sleeping (or, alternately BUMPIN' AND LOVIN' IN THE BED). A bed-buddy's purpose is usually sexual in nature, although s/he may also be a cuddle provider.
"HEE HEE PET PET" is the near-incoherent inner babble of a person who is engaging in pleasurable physical activity (petting) with one of their chosen gender, more than likely for the first time in a long time; this can refer not only to the exclamation but to the activity itself.
Glenda found herself somewhat dazed as Marshall kissed her, the words "HEE HEE PET PET!" running through her sex-starved mind.