You can train the characters you own with lesson points.

The higher rank the character is, the better.

 ▶ Basic > Rare > Rare+ > Elite > Elite+ > Epic > Epic+ > MAX   

      Lv.1 >  Lv.3 >  Lv.6  > Lv.11> Lv.16  > Lv.21 > Lv.26 > Lv.31

To check your characters rank and level, Go to 'Characters' in the main lobby