In Castle Rackrent, Thady Quirk is the narrator, telling the story of the Rackrent family's shortcomings from his own perspective. The storyline, therefore, is influenced by his intentions. As a servant and thus not privy to all the goings-on, his version of events is very subjective.
Excluded from the social class that surrounds him, Thady Quirk is able to involve himself and create a bond, almost a dependence between master and servant, subtlely manipulating situations. As a loyal member of the Rackrent household, he is permitted more indulgences than would otherwise be expected. Thady, effectively, watches his own perceived ascendancy in the ranks of social order. He creates a central character - himself - who otherwise would have no story to tell.
Thady's professed loyalty and the fact that he has "learned the masters' language" mean "honest" Thady can project himself, unnoticed by the self-serving, self-absorbed family. The family's blinkered awareness and belief in social class and his consequent lack of education, prevents them from seeing Thady as anything of influence or importance, allowing him to quietly assert his position. Far better for him to be "respected,"(as far as respected servants go) pretending to "know nothing" than to create a position for himself that prevents or limits his involvement or effect.
Sir Condy, Thady's favorite, is, as a young child introduced to the possibility of being master of the house. Perhaps, not being directly descended, he will be able to escape the character flaws of his predecessors. This allows Thady to almost guarantee that he will indeed have a similar disposition. Thus Thady's position is secure and he basically guides Sir Condy to his unfortunate end. This however makes Thady appear even more the loyal servant. This false modesty and Thady's belief in his own importance allow Thady to continue unnoticed and apparently not complicit in the misfortunes of the family: “little did I think at the time, or till long after, how I was harboring my poor master’s greatest of enemies myself.”
Thady is able to give the appearance of sadness over Sir Condy and he is apparently overcome at the circumstances surrounding Jason's acquisition but at the same time, seeing his son as a "gentleman," makes sense of Thady's subordinate but, in fact, dominant position within the family structure and, by default, the social order.
..."as I have lived so will I die, true and loyal to the family." The reader can make their own decision whether the family he is "loyal" to is the Rackrent's or the Quirks.