Generally,
Defensive fighters: HP & DEF are the most important, followed by Meter Gain and lastly Resistance/Crit Resist (depends).
Offensive fighters: ATK & PIERCE are the most important, followed by Meter Gain and lastly Crit Rate/Accuracy (depends).
HP and ATK are arguably the best stats to have so investing a bit on those is generally good regardless of offense/defense.
Silent Kill is an offensive Valentine. Equip her with ICU and go full on with ATK, PIERCE, METER and CRIT (optional).