Dr. Hawes has aided Sherlock and Joan with their cases. He and Sherlock are regular chess partners. After Hawes was almost killed by an explosion in the morgue, he got addicted to drugs, and Sherlock encouraged him to get help.
After his stint with Mi6, Sherlock returned from London with Kitty (Ophelia Lovibond), his latest apprentice. She and Joan clashed, then bonded. Kitty left after confronting and disfiguring her rapist, fearful of possible arrest.
A restaurateur, Mycroft (Rhys Ifans) has attempted, with Joan's assistance, to improve his often-strained relationship with Sherlock. But when Mycroft, also a covert Mi6 agent, faked his own death, Sherlock took that as a personal affront.
Morland (John Noble), Sherlock's overbearing father, first brought Sherlock and Joan together, as she served as his sober companion. A corporate fixer, Morland took over Moriarty's organization as a way to protect Sherlock.
Both Sherlock's former lover (as Irene Adler) and a criminal mastermind, Moriarty (Natalie Dormer) faked her own death. Sherlock eventually discovered the truth, and the pair still have conflicting feelings and a mutual respect for each other.