The middle bar in blue is Chi
Chi measures internal energies. This secondary ability is influenced by the primary ability, Spirit. You can use Chi to heal yourself, enhance your strikes with Chi a.k.a Chi Attacks, make use of a transformation style or perform magic style attacks. Chi can also be gained from Chi power-ups during combat or a Spirit font.
The Spirit Thief style allows you to steal Chi from enemies during combat.
When healing with Chi, Chi is converted into health at a 1:1 ratio, meaning one point of Chi will heal one point of health.