To build strong character, build your self esteem as in Six Pillars of Self-Esteem, annihilate your ego like the Buddhists, learn to handle adversity for Positive Disintegration, adapt to work with others via Collaborative Literacy, develop your humility (derivative of self-esteem), and continue to grow through life.
From Laws of Human Nature, 4 points of character are:
- How one handles adversity (Ashes and Diamonds)
- Adapt to and work with others (Collaborative Literacy)
- Patience and humility 
- Ability to learn (growth mindset)
And by Jon:
- Add doing something greater than oneself