I don't know if its been mentioned here, but Team Supergirl have cast Rhona Mitra as Mercy Graves.
How much of a role she will play in the season remains to be seen. But it is safe to assume she will be a regular, as leader of the "Humans First" movement, that will feature prominently in the season.
Not sure if she will be the main villain, as Manchester Black is also making an appearance, and will probably fill that role. In addition, Mercy is joined by Otis Graves (yes, that "Otis"), who is described as having "savant-like abilities to assassinate aliens" (whatever that means). Both are linked to CADMUS, so we will probably get an explanation on that at some point tbh.
And ofc you also have Red Supergirl.
It also looks like they could be setting up for the introduction of Lex at some point - late this season or maybe next (thinking that far ahead). Personally, I can do without Lex, seeing as they have Lena and Lillian already, both played by fine actresses. But I will be happy to see Mercy in the show
The show just cast Rhona Mitra as Mercy Graves, familiar to Superman fans as Lex Luthor's super strong personal bodyguard.
Looking forward to Season 4 a little bit more now
Well done Team Supergirl
Peace.
/K