There are 5 magic jewels in the game, each one of these jewels will give your max mana a boost.
After picking up the Red orb underground head out and open the chest right outside.
After the scene with Kaede and Yumemaru at the start of the keep, head upstairs and shoot the demons below to save the samurai, now return to the bottom area and pick up the jewel.
In the West area near the bamboo save point open the quiz box.
At the top of the wooden ramps in the East area!
In the quiz box near the final save point in the East area with Samanosuke right next to the boat.