Once you join the student council, you’ll be met with Hidetoshi and start the Emperor Social Link in Persona 3 Reload. Attending meetings is the way to increase its rank, but you also need to know exactly what to say to him.
Persona 3 Reload Emperor Social Link Guide (P3R)
You automatically start the Emperor Social Link on 4/27 when Mitsuru enrolls you into the Student Council (without asking you first, of course). After the first meeting, you meet Hidetoshi Odagiri, who represents the Emperor Arcana. You can join the meetings on Mondays, Wednesdays and Fridays, except on weeks before tests. The Student Council Room is right in front of your classroom, and Odagiri will occasionally stand in front of it.