Communicated by Michel Van den Bergh MSC: primary 20J05, 17B55, 18G50 secondary 18D35, 18D99, 18C99 Keywords: Mal'cev Protomodular and action representative category Split extension classifier Internal groupoid Commutator and centralizerWe correlate existence and aspect of some properties of centralizers with the conceptual notion of Θ-faithful objects. We produce a large choice of examples from standard algebraic situations to topological models, going through less classical cases as the new notions of algebras introduced by Loday (Leibniz algebras, associative dialgebras and trialgebras), or the dual of any boolean topos.