How to build +Def -Hp Shiro

Keep his Steady Stance and give him the Close Defense Seal. Melee fighters will be basically unable to touch him without Emerald Axes.

I agree with the Steady Stance, or Steady Breath if you have an extra Bike you don't want to keep. Also give him Aether and the Quick Riposte skill or seal and he's basically a Blue Ryoma/Brave Ike(Bike) Which is how I built mine and he's awesome! Also, you can try out the deflect missile or magic seals because he doesn't have distant counter and can benefit from them.