Mother was an extremely large infected form, presented once in the game during a bossfight as the result of Blackwatch's pumping of Bloodtox into the subway systems beneath Times Square. The Mother entity consists of one large, bulbous growth reaching nearly two stories high, and three large tendrils.

In the Trailer

Shown in the Brutal Bosses trailer, this creature apparently holds 14 strains of the virus and is shown as being as tall as a building. It seems to have tunneling capabilities as it arrived in Times Square by bursting through the ground. Also included in the trailer was it releasing a powerful red wave capable of sending Alex Mercer and cars flying backwards.

In-Game

In the game, it is revealed that "Mother" is the codename for Elizabeth Greene, and that the Mother entity is what she later transforms into. It appears after the military pumps a large amount of Bloodtox underground at Times Square, forcing it to surface.

In order for the player to damage Mother, they must first dispose of its tendrils by damaging them. Once the tendrils have been dealt with, the player has a short time to attack Mother's bulbous growth directly; during this time it cannot protect itself.

After a short period, Mother rears up out of reach then burrows into the ground for around a minute while sending up a large amount of health orbs. Once she resurfaces she will have all of her tendrils again. The player must again defeat the tendrils, repeating the cycle until Mother is beaten. Each time the tendrils resurface, they have more health than they previously did. Additionally, every time Mother resurfaces on hard+ mode, she will spawn several hunters and four hydras - curiously enough, these hydras have incredibly low health compared to their counterparts, and drop much less health orbs when defeated.

After a long and arduous fight with Mother, Alex finally defeats the monstrous growth, which relents and releases Elizabeth Greene, who is quickly beaten to a bloody pulp and consumed by Alex.

Defeating Mother

Despite the intimidating appearance of Mother, it can prove to be a surprisingly easy boss when faced with a sound strategy, of which there are several.

1. Groundspike Graveyard/Critical Pain Strategy - This strategy assumes that the player has access to the Groundspike Graveyard and Critical Pain Devastator attacks. Having their Air equivalents is recommended but not necessary. First, the player should build up the maximum amount of Critical Mass health by consuming the abundant supply of local Infected, preferably from a side street protected from Mother's attacks. Once at full health, use the Groundspike Graveyard in close proximity to Mother's base. Having the Air version will make this task far simpler. The Tendril Barrage can be used as a substitute, but it is far less effective. One Groundspike Graveyard is often enough to knock off nearly half the combined health of the tendrils. Repeat this pattern until they are destroyed. If possible, try to use conventional attacks to finish off the last bits of tendril health while keeping a full Critical Mass bar. When all three tendrils are destroyed, Mother's primary growth will descend to the street and flash, indicating vulnerability. Target it and use the Critical Pain. This will deal significant damage, and can be done about three times at most before she retreats. Having a full and fully upgraded Critical Mass will ensure that all three hits can be achieved.
2. Normal Powers Strategy - If it becomes necessary to fight Mother's tendrils without Devastators, there are several means with which to do so. Players should use hit-and-run tactics to strike Mother's tendrils and then flee to avoid damage or consume nearby foes to recover health. Any power can deal damage, but most leave the player vulnerable to counterattack. The long-range attacks of the Whipfist and large area-of-effect damage of the Hammerfists make them the most ideal choices. Armor is essential to reduce the damage that will be taken in doing this. Alternatively, a charged Blade Air Slice from a nearby building will allow for a reliable means of dealing damage, though it will allows the tendrils to whip the player for a large amount of health. Finally, the Rolling Cannonball attack, augmented by the Musclemass, can cause great damage and allow the player to escape before Greene retaliates. When all of Mother's tendrils are disabled, the player should switch to the Blade or Hammerfists power and attack the main mass until it retreats underground. The player should collect the health orb pickups left behind and make ready to repeat the assault until Mother has eventually been defeated.
3. Tank Strategy - This is arguably the easiest strategy. After forcing her to retreat the first time, Tanks will show up. Steal one and start firing on her tendrils until they're defeated, then fire on the main mass. Steal new Tanks as needed.
4. Whipfist Strategy - Using the Whipfist, keep jumping in a circle around Mother, continuously using a fully charged Whipfist attack every time you get into the air. Stop to consume someone when needed, and repeat.
5. Hammerfist Strategy - Once the fight starts, activate your Armor and Hammerfists and go to the top of the building with the big blue ball. Once on top, target Mother and sprint/jump off the building and while in midair, use the Ground Slam ability while Mother is targeted. Doing this will quickly bring down her armor and she will be vulnerable for attack. When she is vulnerable, keep hitting her with the basic Hammerfist attack. Then repeat the process every time she gets up.

When combating Mother directly, it is vital that the player be aware of what Mother is doing when in close proximity to it. Mother's distanced attacks can be quite lethal if allowed to fully connect, and her crimson energy wave attack will do an extreme amount of damage to the player. Though it cannot kill the player on its own, the nearby Military, Hunters, or Infected will often finish the job if the player is not quick to retreat. It should be noted that, though unlikely, if the player has not purchased the Adrenaline Surge upgrade, the shockwave attack will be instantly fatal instead.

Attacks

Mother has several attacks, with which players should familiarize themselves.

• Rock Stream - Mother will fire a far-reaching stream of debris at the player. This stream can stun/interrupt actions and causes light to moderate damage depending on how much of it strikes the player. Mother tends to do this when Alex attacks from greater ranges, particularly when he is using vehicles.
• Greene Orbs - Mother will spawn five large green orbs around itself and launch them at the player. These orbs will track the player until they hit or are avoided. Alone, each projectile will do relatively light damage, but each has the ability to stun/interrupt the player, which can lead to the impact of one or more of the others. Multiple impacts can quickly add up, and full impacts from all five orbs can do moderate to heavy damage. 
• Greene Orbs (Close) - When in close range, Mother can - in addition to her other attacks - fire eight orbs directly from her 'mouth' at the player. These orbs behave identically to the distant Greene Orbs, however there is less room to avoid them, there is no delay before they begin tracking, and there are more of them.
• Tendril Sweep - When the player is in close proximity to Mother, particularly under its main mass, it may assault the player with one of its tendrils, causing light to moderate damage and knocking them back. It should be noted only the two side tendrils are capable of direct attacks; attacking the central tendril will simply result in Mother charging a Crimson Shockwave.
• Crimson Shockwave - Intermittently, Mother will unleash her most dangerous and powerful attack. The screen will become grayscale for a few moments, Mother will scream, and a large, dark red wave of biomass will emanate from her base. This wave will travel outward along the adjacent streets for a fair distance, laying waste to most common enemies caught in its path. This attack will immediately reduce the player's health to critical regardless of their current amount (if the player has not purchased Adrenaline Surge, it will be instantly fatal), but can be easily avoided by a charged jump, ascending a nearby building, or quickly moving away from the wave. It is preceded by yellow energy flowing up from Mother's spine into her head. Fighting in close proximity to Mother over an extended period generally triggers it, and she will perform the attack each time she emerges from beneath the ground.

It could be stated that MOTHER's "Crimson Shockwave" attack is actually Elizabeth Greene's form of a Devastator. Some similarities between the two are:

• 1. During a Devastator Alex needs to Charge up, then release it. And so does Elizabeth Greene as often seen in the boss fight with her MOTHER form she's often seen turning yellow from the bottom up before releasing the shockwave (her charging sequence).
• 2. Greene's shockwave can drop Mercer's Health and Critical (even when full and completely upgraded) all the way to Adrenaline Surge, a special recovery state right before death (assuming you bought it, otherwise it's an instant kill). Instant kills and severely weakening opponents is a common factor for a Devastator.