An attribute of God.
God loves everything that He has made; there is no creature which the Creator does not love with a real love.
This love urges Him to shower benefits upon them.
When any of His creatures is in need, and more so when His rational creature, man, is in need, and most of all when he is in dire need and misery of having offended his God, then this love of God is not withdrawn; even then is He willing to shower His benefits upon him, He is willing to forgive him, and this love and goodness of God we call His mercy.
New Catholic Dictionary