Once your Saint reaches is current level cap, you have to increase its Star level by Enlightening it, before you can level it up further. Enlightening increases a Saint’s Star level by one. Each Saint can be Enlightened 4 times; this also increases its basic attributes.
At Saint level 20: It costs 2 Growth Tomes or 2-Star Saints to go from 2 Stars to 3 Stars.
At Saint level 40: It costs 3 Growth Tomes or 3-Star Saints to go from 3 Stars to 4 Stars.
At Saint level 60: It costs 4 Growth Tomes or 4-Star Saints to go from 4 Stars to 5 Stars.
At Saint level 70: It costs 4 Growth Tomes or 5-Star Saints to go from 5 Stars to 6 Stars.
After a Saint reaches its level cap, head into the Intensify Saint menu by tapping Enlighten in the Functions menu on the left-hand side.
The quality of Saints used for Enlightenment does not matter, the only restriction is their Star level. It is usually best to use C-class Saints and other less valuable Saints as materials. Growth Tomes can also be used, and they are on the whole, cheaper than using Saints. They are, however, more difficult to acquire, and as for where to get them, please refer to the “How to Obtain Tomes” guide.