A person who is obsessed with knives is often referred to as a "knife enthusiast" or a "knife collector." However, if their obsession with knives becomes extreme and is associated with dangerous or harmful behavior, they may be described as having "knife fixation" or "knife fetishism." It's important to differentiate between harmless enthusiasm for knives, such as collecting or appreciating their craftsmanship, and any behavior that may pose a risk to themselves or others. If someone's fascination with knives appears to be harmful or dangerous, it's essential to encourage them to seek professional help or counseling.