A figure’s skills are always in effect while it is on the battle map.
FLANK—Enemies within 2 spaces of this figure are -1 DEF from brawl attacks made by other AGN figures.
The figure with Flank doesn't have to participate in the attack, just be within 2 spaces. So a unit with Flank can help a monster making a brawl attack and vice-versa. A well placed Unit with Flank can benefit multiple attacks during a round.
A skill's description will say "If this figure participates in an attack..." if it needs to participating in an attack for the skill to be in effect.
Triggers can only be used while making an attack, and usually only the Triggers of the figure leading the attack.