Sarah Kerrigan is the primary hero unit of the StarCraft II: Heart of the Swarm campaign. She initially appears very human, and is armed with a rifle. After the first three missions of the campaign are completed, she leaves her rifle after a while and resorts to shooting psionic shockwaves from her hands.
In Heart of the Swarm, the campaign operates similarly to an RPG. Kerrigan focuses on increasing her personal power through mutations while the zerg protect her.
Kerrigan leads from the front. She can die and "respawn" endless times in most missions at a hatchery: the exceptions are, obviously, when there is no hatchery available to a player and if the primary hatchery is destroyed. Kerrigan's revival always takes 60 seconds.
Kerrigan is controlled as a hero unit in "The Infinite Cycle" in Legacy of the Void. She is identical to her fully levelled hero unit in Heart of the Swarm, with the abilities Kinetic Blast, Chain Reaction, Spawn Banelings, and Fury.
In numerous missions, the player will be given optional objectives that will level up Kerrigan. As Kerrigan levels up, her stats increase and she gains access to more abilities. At maximum level, Level 70, Kerrigan has 800 HP, deals 50 damage, has 110 maximum energy, and 3 armor.
After The Crucible, she becomes the Primal Queen of Blades, resembling her old form. After this transformation, Kerrigan unlocks a third column of abilities for each tier, and her maximum energy decreases to 100, but her energy regeneration rate substantially increases. The transformation also causes her to instantly gain 10 levels.
Kerrigan's abilities may be altered between missions. Only one ability from each tier may be active at once.
Some light tactical analysis will be given for the abilities of each tier, which may help players in deciding what to use in each tier.
All three of these are useful in different situations. Kinetic Blast is useful for quickly killing or weakening powerful enemy units, Heroic Fortitude greatly increases Kerrigan's durability, and Leaping Strike allows her to deal damage and move to a target location quickly, and can be used to jump up and down cliffs. Beyond the cliffjumping ability though, Leaping Strike is functionally very similar to Psionic Shift, so the player can choose one or the other based on what is needed - singular damage or area damage.
Crushing Grip is useful when Kerrigan is accompanied by friendly units, and can disable an enemy party for other units to close it. The other two are more useful when Kerrigan is operating independently. Chain Reaction can be very useful when paired with Infest Broodlings, and Psionic Shift is a good area-of-effect damage skill useful for quickly killing masses of enemies.
Primary hatchery tracks how many zerglings have died. When ability is activated, begins 30 second timer and respawns up to 10 killed zerglings at end of timer. Zerglings killed beyond the 10th count toward next usage(s) of ability. If primary hatchery is destroyed, timer continues from another available hatchery.
Zergling Reconstruction's use is dependent on player style, namely the proliferation of zerglings in an army. Automated Extractors are useful for maps where the player is heavily vespene dependant, as the player can mine geysers without needing a nearby hatchery. Improved Overlords is useful for maps where the player is reliant on speed, allowing the player to grow their army much more quickly with the ability to rapidly increase supply.
Wild Mutation has minimal applications on maps where Kerrigan is forced to solo, but can be very powerful on maps where the player has an army. Mutalisks particularly can be stacked so they all received the boost, then make a quick surgical strike before it wears off.
Spawn Banelings is an excellent scouting and harassment ability; from the instant a mission starts spawn them, Attack Move, and keep doing this every chance you get. You'll gain valuable intelligence on enemy positions and they will do damage to enemy encampments no matter how they've been upgraded, making it significantly easier on your forces when you launch a main attack. The spawned Banelings also benefit from standard Ground troop armor and melee weapon upgrades, the latter of which quite heavily upgrades the damage that this ability can do.
Mend is outperformed by Spawn Banelings (with the Regenerative Acid and Ability Efficiency upgrade); both by being able to grant more healing per use to your forces and still be useful offensively to attack enemy forces.
Twin Drones is very useful in most missions, rapidly increasing the player's economical strength early. Malignant Creep is useful if the player is spreading creep rapidly, particularly in missions where creep spread may be needed for some purpose, and on missions where more defensive gameplay is needed. Vespene Efficiency is best used if the player anticipates a high reliance on vespene for a mission.
Infest Broodlings is useful when paired with Apocalypse and Chain Reaction. Apocalypse in particular will likely kill large numbers of enemy units, giving the player an army of broodlings to unleash on any survivors. Fury is a good choice if Kerrigan is expected to be a mainly solo frontline fighter, almost doubling her damage output, and Ability Efficiency is excellent if Kerrigan is expected to be more of a "mage"-like character supporting an army spamming her abilities.
Three ultimate abilities in three different flavors: Nuke, summon one massive hero unit or summon an army. Apocalypse is the ideal choice for crushing enemy bases. Between the other two, the sum total damage output potential of the drop pods army is greater than that of the leviathan, but also note that the drop pod units are much easier to kill to reduce that damage output quickly.
Note that all Tier 7 abilities cost 100 energy and start on a cooldown, meaning a player have to wait 5 minutes to use them (or 4 minutes with Ability Efficiency).