Jess is a Green Earth CO with slight boosts to land vehicles, but penalties for every other unit type.
Day-to-day Effects: All of Jess' Land Vehicles have 10% increased offense. All of Jess' Foot soldiers, Air and Naval units have 10% decreased offense.
Power: Turbo Charge -- All of Jess' units have their Ammo and Fuel fully replenished. Jess' Land vehicles gain 20% increased offense, 10% increased defense and +1 Movement. All other units gain 10% increased offense and defense.
Super: Overdrive -- All of Jess' units have their Ammo and Fuel fully replenished. Jess' Land vehicles gain 40% increased offense, 10% increased defense and +2 Movement. All other units gain 10% increased offense and defense.
First things first: never bring Jess to an infantry fight. She'll just lose, ok?
Dame Jess's tanks (and artillery, and every other land unit that can fire, really) are poetry in motion (according to Javier), however; ignore them to your peril.
She's therefore suitable for tank rushes and all-out attacks with direct units (almost the same as Max's style, really), but can resort to indirect battle (which Max is hard-put to utilize, anyway) if needed. She can also utilize stealthy units like Submarines and Stealth Planes effectively (despite the attack reduction) due to the refuelling effect of her powers.
Air units can be deceptively useful on Jess, even with 10% reduced attack; they can be used to attack land units without much retaliation, and will usually force opponents to build Anti-Airs, rendering their land force more vulnerable to your tanks. Don't go relying on air units or superiority to win your battles, though, and engaging on air-to-air fights will only get you behind. Make Anti-Airs for enemy air units whenever it's reasonable to do so.
Take note that even though Jess's infantry units have reduced offense, their defense is normal, allowing Jess to create meatshields for indirect combat. Just beware of infantry attacking your shields, and never think of breaking enemy meatshields with equal numbers of infantry.