Poptropicans are a virtual equivalent of humans that inhabit Planet Poptropica. Your avatar is a Poptropican.
In Poptropica, Poptropicans are small, human-like people that can run fast, jump high, and swim. Their heads float slightly above their bodies, similarly to a bobblehead. Virus Hunter Island reveals that Poptropicans do have necks.
Poptropica Worlds changes the design of Poptropicans to look more like normal humans, giving them visible necks and more realistic proportions. These Poptropicans wear shoes at all times.
In the comic Zomberry: Day Zero, explaining why the Zomberry outbreak began, Poptropicans are depicted with normal sized arms, hands, and normal legs. Many Poptropicans do not wear shoes, although they can if they choose, as shown on Wimpy Boardwalk Island and the Running Shoes store item.
Many toys, wallpapers, and other evidence seen on islands prove that Poptropicans, when not shown with simple circular hands, have four fingers. This is supported by the fact that Poptropicans have fingerprints (Spy Island, Mystery Train Island), and the fact that Joe Stockman has visible fingers on Virus Hunter Island. However, Back Lot Island and a Pop Quiz, it is mentioned that Poptropicans do not have thumbs. It may be possible that while Poptropicans have fingers, they lack opposable thumbs.
Poptropicans are able to withstand physical pain, including getting electrocuted (Super Power Island), jumping off tall buildings (Super Power Island), and surviving major crashes (Cryptids Island). They also seem capable of squeezing into tight spaces (Wild West Island), and are squished when compressed (Lunar Colony Island, 24 Carrot Island). Poptropican ghosts do exist, as shown on Ghost Story Island and Nabooti Island, appearing similar to normal ones, except glowing and slightly transparent.