When choosing a hair light, if your main light is a spotlight, your hair light should also be a spotlight. If your main light is a softer light, then your hair light should also be softer, as the hair light is a continuation of the main light.
A soft main light is like a cloudy day, so if you use a spotlight for the hair light, how did it become a sunny day on the hair? If the subject has blonde hair, use a softer hair light with diffusion on it. If the subject has black hair, take the diffuser off and let more shine out of the light.
Place the hair light opposite the main light. Its job is to give detail to the hair and separate the hair from the background. I use a strip bank because it lights the top and sides of the hair when tilted on a 45° angle toward the hair. Note: Don’t use hair lights for bald people. Hair light is for hair!