To lead that off, I'll start with the person I think deserves to be leader of the second tier of characters: Gordon.
Name: Gordon Mayes.
Connected to: Anthony.
About Gordon: While he started out as someone Elly saw as a bad influence because she needed someone to blame for the fact that Michael was born an entitled, grandiose and irrational idiot, Gordon has matured into the tough friend they all need from time to time. He's there to keep Anthony on the straight and narrow, to keep Liz from kicking herself too badly for her part in what everyone calls The Great Dysfunction and to reassure Elly that she at least tried to get raising Michael right. His long-term character arc is to put an end to the conspiracy to shield Mike from the reality that he's just a wide boy. His greatest ally in that fight is the relative most of the adult Caines and Pattersons are nervous around: April.
His secondary assistant in that battle is his buddy Lawrence who still resents the fact that Mike essentially destroyed any real closeness with his neurotic mess mother Connie because the stupid jerk thought (and still sort of thinks) that Lawrence was laughing at him behind his back pretending to be straight instead of being terrified of how people would react.