McFarlane introduces himself as the "unfortunate" John Hector McFarlane, and it's a rather apt description. This poor dude gets framed for a murder by a guy named Jonas Oldacre, who has been nursing a grudge against McFarlane's mom. McFarlane is a young professional, a solicitor (lawyer), and his profession is used against him by Oldacre. Jonas not only frames McFarlane for murder, but he also lies and makes McFarlane think he's going to inherit a bunch of money in Oldacre's will. This gives McFarlane a "motive" for the murder. Fortunately, McFarlane got Holmes on the case, and his name was cleared in the end.
This old man has one heck of a vindictive streak. McFarlane's mom dumped him back in the day, so Oldacre decided to get his revenge by framing her son for his murder. He sets McFarlane up with a fake will, then stages his death and hides out in his own house until Holmes flushes him out with a fake fire. After Oldacre emerges, he tries to pass the whole thing off as a "practical joke" (Norwood Builder.189).
Mrs. McFarlane is John Hector's mother, and she partly causes the whole messy case because she dumped Jonas Oldacre once upon a time. And no one dumps Jonas Oldacre, no one! Of course, he waited twenty plus years for revenge. That's how Jonas rolls apparently. You never know when he'll strike in an overly-elaborate fashion. Mrs. McFarlane is a sympathetic character and she is also a handy information tool who gives us important information about Oldacre and his motives.
This is the man to whom Watson sells his medical practice after Holmes returns from the dead. This is an important detail about Watson's character. He gives up his own profession in order to live with Holmes and follow him around solving crimes.
This is Oldacre's housekeeper, who totally stonewalls Holmes when he tries to question her. She presumably knew what her mean boss was up to the entire time, but she didn't spill the beans.