Crestfallen Studios

Firstly, I have Lexus selected as a member of my Party, but she won't ever fight. no matter what fights I enter into she's never coming up as one of the combatants (despite being selected as a member of my party).

I can select Slade/Midian/Sacrifica however, and work around.

Try first removing everyone from your party, then put your members in. That should work.