What does frederick mean by "mr. covey's forte consisted in his power to deceive"?

hm, switch the relative; nominative and indefinite; objective ? not too bright in the knowledge of the english.

the narrator is saying that the old lady made her feel uncomfortable. you know this because they say "i found that rather irksome", "perhaps it was her being so upright and trim", and "i don't think it was that, because i thought that quaintly pleasant." hope this !