Another member of Lex Luthor's inner circle is joining Supergirl. The Strain's Rhona Mitra has been cast as Lex Luthor's bodyguard, Mercy Graves. She's joining other Supergirl newcomers, such as Sam Witwer as Agent Liberty, David Ajala as Manchester Black, and Robert Baker as Mercy's brother, Otis Graves.
Supergirl giving Mercy a brother - something she's never had in the comics - does suggest the series will be changing her a bit. Mercy's character will still be known on Supergirl as Lex Luthor's former bodyguard. However, it appears Supergirl's version of Mercy will be stepping out from behind her former employer's shadow. Rhona Mitra's Mercy Graves is her own woman.
Otis, meanwhile, will be Mercy's right-hand man and muscle. He isn't intelligent but he has "savant-like abilities to assassinate aliens." That's pretty jarring character description for a show as family-friendly as Supergirl, but it does get the point across. Otis is a completely original character. Mercy Graves was first introduced into DC canon during Superman: The Animated Series. Initially a background character, Mercy existed to make Lex Luthor appear more intimidating. Eventually Mercy gained more of her own identity and joined the comics too. She's never become quite as popular as other DC Animated Universe original characters like Harley Quinn. However, Mercy has a following, and she's been closely associated with her boss, Lex Luthor ever since. Even Batman V Superman, which radically changed Lex Luthor's character, included Mercy Graves. In that movie, Mercy was played by Tao Okamoto.
The connection to Lex is why it's very intriguing that Supergirl has decided to add Mercy to the series. Although the CW series has introduced their own version of Superman, played by Tyler Hoechlin, Lex Luthor has been almost entirely off-limits. The most that has been seen of Lex was during a quick flashback scene in season 2. Lex Luthor's shadow has loomed heavy in Supergirl, especially on his sister, Lena (who's been a recurring character since season 2). Yet, so far the role of an adult Lex hasn't been cast. Rhona Mitra's casting as Mercy likely confirms that Supergirl will never introduce Lex Luthor properly. The series seems more content to introduce the women around Lex (his mother, his sister, his bodyguard, etc.) than the man himself. This is likely the right move for Supergirl. It fits the series' own motif of female empowerment and representation. However, there might always be a Lex Luthor-shaped hole in many Supergirl fans' hearts.
Supergirl season 4 premieres October 14 on The CW.