The hepatic stellate cell is a connective tissue cell that, in its quiescent state, helps to regulate vitamin A metabolism (storage, release, etc.). However, it is constantly surveying its environment and, during liver injury, transforms into a myofibroblast phenotype, and participates in the homeostasis of liver extracellular matrix, repair, regeneration and fibrosis. It is the major source of type I collagen in the fibrotic liver.
Beyond these features, hepatic stellate cells have been implicated as regulators of hepatic microcirculation via cell contraction, and in disease states, in the pathogenesis of intrahepatic portal hypertension. Proliferation and migration of these cells and expression of chemokines are involved in the pathogenesis of liver inflammation and fibrogenesis.
Bear in mind that the stellate cell is not the only cell involved in hepatic fibrogenesis; there are also resident (portal, lobular) fibroblasts, bone marrow-derived fibroblasts, and cells involved in EMT (epithelial-mesenchymal transition, those involved in biliary fibrosis). In the image above, you can see some activated stellate cells in culture. In vivo, they mostly reside in the hepatic sinusoids.