Searching these names briefly in the CK2 folder has revealed the following
nick_the_lawgiver = {} never appears ingame
nick_the_usurper = {} given to a historical Welsh character and a Danish character
nick_the_hammer = {} historical Frankish character
nick_the_conqueror = {} several historical characters + received via the conquer casus belli
nick_the_victorious = {} a swedish character
nick_the_crusader = {} danish, occitan character
nick_the_holy = {} become paragon of virtue objective, also castillan and danish character
nick_the_avenger = {} castillan character
nick_the_brave = {} castillan, polish characters
nick_the_fearless = {} occitan character
nick_the_martyr = {} saxon character
nick_the_unlucky = {} not used
nick_the_lucky = {} not used
nick_the_restorer = {} basque, polish characters
nick_the_effeminate = {} given via some events
nick_the_clueless = {} events
nick_the_ill_tempered = {} not used
nick_the_affable = {} frankish character
nick_the_heathen = {} swedish character
nick_the_liberator = {} swedish character
nick_haardraade = {} norwegian character
Hope this answers your question.
nick_the_lawgiver = {} never appears ingame
nick_the_usurper = {} given to a historical Welsh character and a Danish character
nick_the_hammer = {} historical Frankish character
nick_the_conqueror = {} several historical characters + received via the conquer casus belli
nick_the_victorious = {} a swedish character
nick_the_crusader = {} danish, occitan character
nick_the_holy = {} become paragon of virtue objective, also castillan and danish character
nick_the_avenger = {} castillan character
nick_the_brave = {} castillan, polish characters
nick_the_fearless = {} occitan character
nick_the_martyr = {} saxon character
nick_the_unlucky = {} not used
nick_the_lucky = {} not used
nick_the_restorer = {} basque, polish characters
nick_the_effeminate = {} given via some events
nick_the_clueless = {} events
nick_the_ill_tempered = {} not used
nick_the_affable = {} frankish character
nick_the_heathen = {} swedish character
nick_the_liberator = {} swedish character
nick_haardraade = {} norwegian character
Hope this answers your question.