Website and data research by 101Leafy.
Lightning Reflexes

If unit attacks first, and AS ≥ foe's AS+10, unit's follow-up occurs before foe's counter during combat.
Fire Emblem Fortune's Weave Wiki by 101Leafy
If unit attacks first, and AS ≥ foe's AS+10, unit's follow-up occurs before foe's counter during combat.