Your beauty should not come from outward adornment, such as braided hair and the wearing of gold jewelry and fine clothes. Instead, it should be that of your inner self, the unfading beauty of a gentle and quiet spirit, which is of great worth in God's sight. (1 Peter 3:3-4 NIV)
What is the inner self? Or as the King James Version states, “the hidden man?”
The word hidden in the Greek is kruptos meaning “hidden, concealed, secret” according to Strong’s, The Exhaustive Concordance of the Bible. Another source, Matthew Henry’s Commentary, states the inner self is our soul.
We can dress ourselves up to look great on the outside. Other people cannot see what is on the inside. What is hidden is what cannot be seen. We can act like we have character, but what is really inside? The Lord knows what is in the inner person. We’re to work more on adorning the inner person with godly character than adorning the outside with ornaments that can be lost or destroyed. Do we behave the same way inside our homes as we do out in public? Here’s what Matthew Henry describes for us about how to adorn the inner self:
The ornament prescribed. It must, in general, be something not corruptible, that beautifies the soul, that is, the graces and virtues of God’s Holy Spirit. The ornaments of the body are destroyed by the moth, and perish in the using; but the grace of God, the longer we wear it, the brighter and better it is. More especially, the finest ornament of Christian women is a meek and quiet spirit, a tractable easy temper of mind, void of passion, pride, and immoderate anger, discovering itself in a quiet obliging behaviour towards their husbands and families. If the husband be harsh, and averse to religion (which was the case of these good wives to whom the apostle gives this direction), there is no way so likely to win him as a prudent meek behaviour. At least, a quiet spirit will make a good woman easy to herself, which, being visible to others, becomes an amiable ornament to a person in the eyes of the world. 
Let’s adorn the inner self by the grace of God and the Holy Spirit by whom we’re sealed.
Henry, Matthew: Matthew Henry's Commentary on the Whole Bible : Complete and Unabridged in One Volume. Peabody : Hendrickson, 1996, c1991, S. 1 Pe 3:1