Who is Mr. Frankland in The Hound of the Baskervilles?
In any mystery, it is important to get to know the suspects. In this case, the neighbors!
The moor is very sparsely inhabited, and those who live near each other are thrown very much together. For this reason I saw a good deal of Sir Charles Baskerville. With the exception of Mr. Frankland, of Lafter Hall, and Mr. Stapleton, the naturalist, there are no other men of education within many miles. (enotes etext pdf p. 13)
Mr. Frankland lives in Lafter hall (notice the pun on "laughter"). He is basically a muder suspect, though he is described as “an unknown factor” in chapter 6. Mr. Frankland is a neighbor. It is noted that there are not many educated men around. He is ”an elderly man, red faced, white haired, and choleric” (p. 55). He loves the law, and enjoys wasting his money on lawsuits.
He fights for the mere pleasure of fighting and is equally ready to take up either side of a question, so that it is no wonder that he has found it a costly amusement. (p. 55-56)
Thus Mr. Frankland is described as the comic relief. He keeps things interesting! It is also noted that he has a telescope, and a daughter who wrote to Sir Charles but never got his interest. You could consider Mr. Frankland the comic relief, or a red herring (a clue put in to confuse the reader).