This study introduces a new sketch-based hairstyle authoring interface for virtual characters captured using multi-view stereo. Individual hairs are typically modeled by interpolating guide hairs, which are downsampled hair strands representing the overall hair shapes. Unlike conventional geometric modeling, we incorporate the physical properties of hairs, such as gravity, collision, and bending resistance, in guide hair modeling. The use of physics-related shape optimization in the interface allows natural-looking 3D hair shapes to be modeled from minimal user specifications. We provide an interactive sketch tool that allows the user to specify hair shapes from multiple viewing angles. Our interface generates feedback regarding how much a designed sketch conforms to the laws of physics, enabling the user to strike a balance between artistic intention and physical naturalness. We further introduce an automatic sketch suggestion system based on the hair orientation obtained using image filtering.