NE′MESIS (Nemesis), is most commonly described as a daughter of Night, though some call her a daughter of Erebus (Hygin. Fab. Praef.) or of Oceanus (Tzetz. ad Lyc. 88; Paus. i. 33. § 3, vii. 5. § 1). Nemesis is a personification of the moral reverence for law, of the natural fear of committing a culpable action, and hence of conscience, and for this reason she is mentioned along with Aidôs, i. e. Shame (Hes. Theog. 223, Op. et D. 183). In later writers, as Herodotus and Pindar, Nemesis is a kind of fatal divinity, for she directs human affairs in such a manner as to restore the right proportions or equilibrium wherever it has been disturbed; she measures out happiness and unhappiness, and he who is blessed with too many or too frequent gifts of fortune, is visited by her with losses and sufferings, in order that he may become humble, and feel that there are bounds beyond which human happiness cannot proceed with safety. This notion arose from a belief that the gods were envious of excessive human happiness (Herod. i. 34, iii. 40; Pind. Ol. viii. in fin., Pyth. x. 67). Nemesis was thus a check upon extravagant favours conferred upon man by Tyche or Fortune, and from this idea lastly arose that of her being an avenging and punishing power of fate, who, like Dike and the Erinyes, sooner or later overtakes the reckless sinner (Apollon. Rhod. iv. 1043; Sophocl. Philoct. 518; Eurip. Orest. 1362; Catull. 50, in fin.; Orph. Hymn. 60).
A nemesis is an arch enemy, or in other words, the person who is always standing against you whenever you are trying to do anything. Not always the bad guy, as both the Joker and Batman are each others nemesis.