Each character stat has a level cap that depends on rarity. The maximums are as follows.
5★ Lv100 4★ Lv90 3★ Lv80 2★ Lv70 1★ Lv60
Once you raise a stat to its maximum level, you can use Level Unlock inside Unlock Potential to spend Six-Flower Orbs and raise that level cap.
The battle log for the selected character scrolls here
Levelling up raises the required EXP for all four stats at the same time, so level up with care.
Press and hold a Feature icon to see its details.