A Doppelganger (or Doppelgänger by German spelling) is a German-based monster who can transform from anyone they choose to. Their name literally means "double-walker" or "double-goer" in the German language.
Bite Size Monster Dictionary
A creature known in literature to look exactly like someone else. Anyone who sees this creature typically dies within a few days, and it identity remains a mystery. In German the name means "one who walks double".
Their appearance, as previously mentioned, is highly variable. Trying to state all their possible forms is thus obviously impossible. They do have a tendency to prefer hominid guises of about 6 feet (1.8 m) in average height.
Powers & Abilities
It can copy the physical form of anyone it comes into contact with. While in the copied form, it can use any and all physical powers the original form possesses. It has been shown in Rosario + Vampire II Chapter 006 that Doppelgängers can combine copied forms to gain access to all the forms' powers. However, this copying ability is not perfect, since the Doppelgänger in question did not mimic Tsukune's Ghoulish state after copying his form (the copy's appearance was perfect, albeit with the Holy Lock not included).
This would mean that the mimic ability copies forms but not mindsets or other nonphysical traits.