You are going to need all the health you can muster in order to beat the more difficult portions of Ninja Gaiden 4. The upgrades for that attribute are scattered throughout the course of the storyline and require some out of the way exploration as well as the completion of side content and a large part of New Game Plus.
This Ninja Gaiden 4 guide lists the locations of all the items, including the Black Scales and Phantasmal Herbs, needed for health upgrades for Yakumo.
Yakumo Health Upgrade Locations
Chapter 1
Phantasmal Herb #1
As a reward for completion of Mission 2.
Chapter 2
Black Scale #1
Go left from the monorail track after the battle with the drones and flying enemies.
Acquiring it will trigger the creation of your first Dragon Lord’s Grace through the automatic combination of the prior Phantasmal Herb and this Black Scale.
Chapter 3
Black Scale #2
As a reward for completion of Mission 4.
Phantasmal Herb #2
In a chest at the top of a stairway, after the Burabura Lantern Daemon fight.
Chapter 4
Black Scale #3
In a chest atop a wall that you will have to run up to reach. Find this location by taking the left route at the fork after the stealth portion at the start.
Black Scale #4
As a reward for completion of Mission 8.
Chapter 5
Phantasmal Herb #3
As a reward for completion of Mission 11.
Phantasmal Herb #4
As a reward for completion of Mission 12.
Chapter 6
Black Scale #5
After landing on the rope bridge, make an about face instead of proceeding to the objective. Find it in a chest to the left of this area.
Chapter 8
Black Scale #6
As a reward for completion of Mission 15.
Chapter 9
Black Scale #7
As soon as the surf sequence begins, bear left after ducking under the glowing gate. Remain on the left until you reach the combat area with the two chests, the right of which holds the Black Scale.
Phantasmal Herb #5
As a reward for completion of Mission 17.
Chapter 11
Phantasmal Herb #6
In one of four chests found at a station, post-completion of Mission 19.
Chapter 12
Phantasmal Herb #7
In a chest just off to the right at the top of the short stairway, prior to the red laser hallway section.
Black Scale #8
As a reward for completion of Mission 22.
Chapter 19
Phantasmal Herb #8
In a chest to the left of the shaft you will ascend, post-battle versus Ryu.
This marks the last of the eight Yakumo health upgrades that can be found in regular New Game. The remaining two health upgrades can only be found in New Game+ by loading a completed save and accepting the newly unlocked missions.
New Game Plus Chapter 2
Black Scale #9
As a reward for completion of Mission 26.
New Game Plus Chapter 6
Phantasmal Herb #9
As a reward for completion of Mission 30.
New Game Plus Chapter 8
Black Scale #10
As a reward for completion of Mission 31.
New Game Plus Chapter 11
Phantasmal Herb #10
As a reward for completion of Mission 33.
Those are the unlock locations for every item required for all the health upgrades possible for Yakumo in Ninja Gaiden 4.