The Kradin were a large humanoid species found in the Delta Quadrant. They had muscular ridges around the face, and teeth on all sides of their mouth. According to the Vori, with whom the Kradin were engaged in a war, they were "beasts" who were capable of a variety of atrocities, rape, genocide, and killing Vori in their sleep.
The Vori furthermore claimed that the Kradin craved air supremacy and fired on any craft that didn't show Kradin colors. They were said to engage in a practice called neck-strapping, and left bodies upturned so they could not enter the Way After. They also claimed that their breath stank of muck, and perhaps figuratively, claimed that there was no heart thumping beneath their flesh and described them as motherless. (VOY: "Nemesis")
However, there was no independent evidence for the Kradin's vicious nature. In fact, when Commander Chakotay of the USS Voyager was kidnapped by the Vori, the Kradin aided in his rescue. (VOY: "Nemesis")
One other detail that was changed between the real Kradin and those in the simulation was that the latter had a modified, less Human voice.
The image of an angry Kradin was among the disturbing thoughts that Tuvok imposed upon the Mari Guill during a mind meld. (VOY: "Random Thoughts")
A Kradin fighter was taking part in Tsunkatse matches aboard Penk's starship. (VOY: "Tsunkatse")