Send a signal to Barton, who will use the Berserker skill. Using the Berserker skill will increase Barton's Debuff Resistance/Speed/P. Atk./P. Accuracy/P. Def./Atk. Spd./Critical Damage. Recovre 20% of HP when killing the opponent. Additionally, P. Atk. +10% and Critical Rate +100 when HP is below 60%. P. Atk. +30% and Critical Rate +300 when HP is below 30%. The whistle disappears after 2 uses.