The Flash/Metahuman

Metahuman Flash is one of the most powerful basic combo attackers. His passive allows him to launch a flurry of unblockable crit basic attacks upon special use at unrivaled speed, taking down his opponents in a flash. He can be obtained by ranking in the top 5% of players during certain Online Battle seasons. Otherwise, he is available in The Flash Starter Pack, which can only be purchased once for $19.99. It also comes with 135,000 Power Credits. He can also be obtained from the TV Pack, but it doesn't directly unlock him for promotions.

Strategy
When Superspeed is active, yellow lightning effects can be seen around him constantly. It doesn't triple his attack speed so much as dealing 3 hits with every attack, with a bonus ~20% crit chance. Black Adam's passive will interrupt him after every single hit for 3 times upon tag in. Power generation is greatly reduced during the passive duration. His normally one hit combo ender will also hit 3 times, allowing him to trigger Gauntlets of Azrael's/League of Assassins Adept Blade's life drain 3 times (Ra's Al Ghul's Scimitar does not apply any power drain on his normal combo enders, and will only apply it once with his passive-enabled 3 hit combo enders). However, for reasons unknown AI opponents tend to be able to block and interrupt his light basic attacks; therefore, it is more advisable to just put 2+ pieces of basic damage/crit on him and mow down your opponents with heavy attacks efficiently.

Both special 1 and special 2 are viable for his passive. Special 1 allows you to use two triple combos. With 2 pieces of basic/crit gear, this should be about enough to knock out a character at similar stats. Special 2 grants a longer passive duration, and a combination of LexCorp Chest Armour V2 and LexCorp Helmet Armor V2 would make him quite tanky and improving his power generation indirectly, which he often struggles with. On the other hand, his special 2 has a large amount of hits which makes it likely for his opponent to gain enough power to retaliate with a special if they are not knocked out.

While his special 1 hits 4 times, should you find yourself in a situation when 4 unblocked hits will give your opponent just enough power to retaliate with an inconvenient special, it is possible to not swipe at all, which will cause it to only hit once, while still activating his passive; the massive damage output of his basics while it is active makes the lost damage insignificant.

Power generation gear can be considered to help him reach his SP1 faster.

Cloak of Destiny may be tempting, but is a gimmicky pick as he needs to use a special first before his passive activates. It is possible to use a special, switch to another character to stun, and tag back in to take advantage of the damage boost, but it will waste much of his passive duration (special 1 doesn't even last long enough), requires too much set up and the stun effect will wear off mid-combo anyway. Its crit boost (combined with 2 pieces of basic damage/crit) does allow his crit basic attacks to hit just as hard as if he had 3 pieces of basic damage/crit however. It would work better if you have crit augments, but you will have a harder time against crit immunity (Green Lantern/Red Son, Batman/Arkham Knight).

Gears that increase crit damage, such as League Of Assassins Adept Hood or LexCorp Helmet Armor V2, may also be considered. Other LexCorp Set pieces can also be a good option, as while the 3-piece effect doesn't fit him well, the 2-piece effect (reversing power drain) is great for him.

His superspeed combos are incredibly fast and deadly. However, because they strike multiple times in a single tap, you cannot stop them to block specials, thus making him vulnerable to specials when attacking. This can also be used to counter him, as right when he is about to attack, use a special attack and Flash cannot do anything to block it since he is in mid-combo.

Good With

 * Any characters that boost power generation, such as Harley Quinn/Animated, or Green Lantern/Regime.
 * Batgirl/Prime: Her one bar of starting power allows him to use SP1 at the beginning of a match.
 * Superman/Red Son: He provides a large damage boost to his already powerful basic attacks. In fact, when combined with Wonder Woman/Red Son, Flash will have a 50% damage boost to his basic attacks and a 30% increase in power generation, allowing him to activate his passive sooner and deal a large amount of damage.
 * Ares/Prime: If both Ares and The Flash's passives activate, then The Flash will have an even easier opportunity at drilling away at his opponents due to their brief inability to block.
 * Batman/Flashpoint: His devastating crits from his passive combines well with The Flash to form a team of vast offensive power.
 * Batman/Prime: His passive gives a 25% increase in DAMAGE, which helps The Flash's passive.

Good Against

 * Reverse Flash: Reverse Flash's passive will not be activated by Metahuman Flash's lightning fast basic combos (unless if he's been hit by his specials first).
 * Batman/Dawn of Justice, The Flash/Wally West Rebirth: Metahuman Flash's superspeed combos do not trigger their Evasion passives.
 * Superman/Injustice 2: While the damage from Flash's special might be negated by Superman's passive, Flash can remove a large portion of Superman's high health with his very powerful basic attacks.
 * Zod/Man of Steel: Since General Zod's passive only lets him take 20% less damage from special attacks and supermoves and does not mention anything about basic attacks, this makes Flash very effective against him.
 * Killer Frost/Prime: Although Killer Frost can slow down his abilities to use specials with her passive, Flash has a near-unrivaled damage output from his SP1, meaning that he can make the most out of 1 bar of power despite her power dampening.
 * Fourth World Set: His basic attacks can effectively neutralize the regeneration effects of the Fourth World Set, countering a large part of their value. Unlike most other characters who can do this, however, all of his specials and basic attacks are multi-hit, which also makes him good at countering the set's resurrection effect.

Countered By

 * Black Adam and Black Adam/Regime: While Flash's passive massively increase his attack speed, Black Adam's passive can interrupt his barrage of deadly basic attacks up to three times upon tag in.Black Adam Regime can also reflect a masive amount of damage if the Flash uses his SP1 to activate his passive against him.
 * Superman/Godfall: his passive allows him to reflect back the damage dealt to him by basic attacks when he blocks. Though the Flash has a chance to use unblockable attacks, he would still recieved back damage upon Superman's blocking.
 * Power drain characters: since Flash's passive is activated upon using his Specials, these characters can quickly prevent him from reaching his full potential.
 * Killer Frost/Prime: Killer Frost slows his power generation with both her passive and special 1, causing him to take longer to reach SP1. Although if his passive is already activated, upon using her SP1 Cold Ice, the Flash would still attack at regular speed.
 * Green Lantern/Red Son, Batman/Arkham Knight: their passives grant immunity to critical hits, negating the increased crit chance of Flash's triple speed combos and decreasing his overall damage output.
 * Nightwing/Regime: Nightwing's passive let him takes 50% less damage from Flash's punishing basic attacks and has a power drain SP1, allowing him to stall Flash from using specials.
 * Sinestro/Antimatter: a possible choice, as Sinestro can force the Flash to tag out with his passive, thus negating the Flash's passive.

Abilities
Here are Flash's abilities.

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

Trivia

 * He's based on Grant Gustin's portrayal of The Flash in The Flash (2014 TV series). His special 2 is named after the season 1 finale, "Fast Enough".
 * His Injustice 2 Mobile equivalent is Multiverse The Flash.
 * Grant Gustin was unaware of the mobile version of Injustice: Gods Among Us or the inclusion of his image as a character prior to being sent a screenshot of The Flash Starter Pack.
 * He is the first character to have their Super Move description completely changed; its description is a reference to a repeated line from The Flash TV series.
 * The name of his passive is very similar to Superman/Prime's "Super Speed" passive.
 * His passive is affected by a variety of glitches.
 * If you activate his special(s), quickly attack the opponent with basic attacks, and then switch in/tag in another character right away, the Flash would still be there for a short period of time, and after he's finished with his "combo", he will switch out. This will also happen if he is knocked out mid combo.
 * During his Superspeed combos, it is possible to activate a special, but not actually performing it at all.
 * AIs are often able to block or use specials even while being hit (unblocked) by his Superspeed combos, particularly his light combos.
 * He was soon brought back only six weeks after his first release due to popular demand. However, the title for the shortest Online Battle season rewards goes to The Joker Unhinged/Suicide Squad, who was brought back only a week after he was first aired.
 * He is potentially the fastest character in the game.
 * He has the same base stats as Sinestro/Antimatter.
 * In the 2.12 update, there is a speed logo that will appear when his passive his active, and disappear when it is not.
 * He is the only Flash character to have uniquely styled lightning effects in his basics and specials.