Each character has two kind of skills, field skill and buddy skill. The effects of those skills vary from boosting attack to restoring health.
Field skills are valid when a character is within other characters' range ( the range of field skill shows up as a blue line. As the once character is closer to the another, the effect of field skill become significant.
Blue lines with white letters indicates the field skills of nearby allies, and red lines with red letters shows the field skills of enemies.