His duty is to solve the universe's entropy problem by forming contracts with girls to become magical girls & fight witches in exchange for a wish. Coldly logical, efficiency-obsessed doesn't understand human emotions, thinks emotions are mental illness. Kyubey's actions seem conspicuously chosen to frustrate those he talks to. If Kyubey is killed, another will step out, eat the corpse & continue to converse. Will never lie if asked, but will omit information intentionally to achieve his goals.