Deeply compassionate — feeling and acting on the misery of others. Used of God's mercy (Luke 6:36: 'Be merciful, even as your Father is merciful') and in the NT exhortation to show mercy.
Luke 6:36 uses oiktirmon in the Sermon on the Plain parallel to Matthew's 'perfect' (Matt 5:48). Mercy, not abstract perfection, is the defining family trait of God's children.
James 5:11 pairs oiktirmon with polusplagchnos (very compassionate) — double-strength mercy language for God. The appeal of Romans 12:1 is grounded entirely in God's mercies, not obligation.