Batgirl/Cassandra Cain

Cassandra Cain Batgirl is the second online battle exclusive character after Ares. Like Ares, she cannot be unlocked, and can only be obtained in limited quantities by ranking at least in the top 5% in online seasons when she's on. Eliting her thus takes a very long time and cannot be bought quickly with any amount of power credits, since she can only be promoted by obtaining multiple promotions of her in Online Battles.

Strategy
The new Cassandra Cain Batgirl is able to knock off 25% of her opponent's current health instantly when tagging in (unaffected by block), making her a true threat in multiplayer. That, and her devastating damage stat which is the highest (tied with Hawkgirl/Regime) that can take advantage of 600 Wonder Woman's passive, being a female character.

The release of Cloak of Destiny added the most terrifying weapon yet to her already formidable arsenal: as Art of Deception stuns opponents at the beginning, they will take vastly increased damage as long as they did not block the first hit, and the crit boost stacks on top of her own crit boost gear for truly absurd amounts of damage. While difficult to confirm due to the low availability of both cards, it is theoretically possible to hit over 200,000 damage with her first special, instantly disintegrating any opposition as long as they are not blocking. This will affect normally stun-immune enemies, like Green Lantern/Red Son, as normal.

However, while she could technically stun with tag in with the cloak, her passive will negate it, allowing her opponent to block.

Cassandra Cain's specials have fast starting animations, making them more difficult to block. Her special 1 is particularly noticeable in that it uses the same name and the same starting animation as prime Batgirl, but is much faster.

If a character tags in at the same time as Batgirl, she will not deal damage. Also, she will not deal damage if a special move is in progress. There is a significant difference between these two cases, however: in the case of double tag, she will gain an unblockable charge that causes her next hit, whether it is a basic attack or a special, to be unblockable, which would likely spell instant death if she is geared properly. This does not apply to the second case. A great way to negate her passive's damage is to use a special when your current opponent is about to be knocked out, so when she tags in you would still be in the middle of the special and would take no damage with no other negative consequences. Most specials have long enough finishing animations to prevent her passive from dealing damage even if the special's last hit knocks out the previous character, but Super Moves do not.

Good with:

 * Hawkgirl/Prime: Her passive lets you tag in more often, as such it's a good strategy to use another character with passive that is activated upon tagging in.
 * Wonder Woman/600: Her passive gives Cassandra Cain Batgirl 30% more damage and power generation.
 * Killer Frost/Regime: Cassandra can easily inflict heavy damage on targets with high health, and Regime Killer Frost severely punishes characters at lower health as it is almost impossible to heal back, making them a very unforgiving combination.

Good against:

 * Scorpion/Mortal Kombat X: As a card with extremely high maximum health, percentage health damage is effective in taking him down. Additionally, she can easily interrupt him with her much faster animations.
 * Batman/Arkham Origins: Assassin's Ambush is also a knock back that cannot be resisted by block; tagging in Cassandra Cain at the beginning of the battle and blocking immediately will almost guarantee that he will use Explosive Batarangs instead of Winged Avenger greatly reducing his threat, especially when he is teamed with Harley Quinn/Arkham.
 * Killer Frost/Prime: Cassandra Cain deals significant damage without using power and has access to crit boosted basic attacks, making her more resistant to power generation dampening than average.

Countered by:

 * Black Adam/Regime: His passive makes it easy to inflict considerable damage in return to her one hit high damage passive which cannot knocks out characters.
 * Hawkgirl/Prime: She snares opponents when she tags in or out, preventing them from tagging in or out.
 * Sinestro: He benefits from enemy tag outs.

Abilities
Here are Batgirl's abilities.

Support Cards and Gear
Here are Batgirl's support cards and gear.

Trivia

 * The 2.6 update added a "Stunned" text to the beginning of her s1, although it already works as a stun for the purpose of Cloak of Destiny prior to that.