Which philosopher's 'categorical imperative' anchors his ethics?

Kant, and the imperative is a test rather than a rule: act only on a principle you could will to become a universal law. Lying fails not because it harms but because universal lying would destroy the practice of promising that makes lying possible - it is self-defeating rather than merely bad.

A second formulation is the one people find more usable: treat humanity, in yourself and others, always as an end and never merely as a means. Merely is doing the work - using a shopkeeper as a means to bread is fine; deceiving them is not, because it treats them as an object to be steered rather than a person who could agree.

Its notorious consequence is that lying is always wrong. Kant defended this against the case of a murderer at the door asking where your friend is hiding, and concluded you may not lie - which almost nobody accepts and which has generated two centuries of argument about whether the system can be repaired. He also, by all accounts, lived so regularly that neighbours in Königsberg set their clocks by his afternoon walk, and never travelled more than a few dozen miles from the city.