True Kree resemble humans almost exactly, with the exception of blue skin. Kree have a higher strength level than that of a human, and require more nitrogen to breathe comfortably[volume & issue needed]. Kree bodies are adapted to environmental characteristics on Hala that are unlike Earth. Hala has higher gravity and a higher concentration of nitrogen in the atmosphere than Earth. Under Earth's lesser gravity, their strength and speed increase. However, they cannot breathe in Earth's atmosphere without using a chemical, "breathing formula", or artificial life-support devices.
The original Kree had blue-colored skin, but a second racial group with pink skin resembling that of human Caucasians emerged over the millennia. The blue-skinned "purebred" Kree have become a small, but powerful, minority. Pink Kree are much more durable than their blue racial brethren.
Adult Kree range (typically) from between 5 to 8 feet (1.5 to 2.4 m) tall. Some female Kree can physically influence men, and a few can even drain the life force of others.
The Kree were an evolutionarily stagnant race. This was due to a single member of the Kree race attempting to gain control of The Crystal of Ultimate Vision. This unnamed Kree found the crystal, but attempted to use it to become akin to a god, with powers as of those of the Phoenix Force. As punishment, the crystal "genetically froze their evolution in place" allowing the rest of creation to pass them by. In an attempt to further their development, some Kree bred with other species, producing the 'pink-skinned' Kree, who are similar in appearance to Caucasian humans. These pinks (also called 'whites') eventually outnumbered the blue-skin Kree, but were far from accepted amongst their brethren; many pink kree were exiled from the homeworld and put into forced labor camps on barren moons by their blue skinned cousins due to prejudice & racism. The Kree value what they consider to be their genetic purity, to such an extent that reproduction outside of the species is a strict taboo. In the Kree empire, it is a crime for a male non-Kree to impregnate any Kree.
The Kree race has produced individual super-humanoid beings, either through natural mutation, genetic engineering, or cyborg technology, including Captain Mar-Vell, Ronan the Accuser, Ultimus, Shatterax, Korath, and others.
The Supreme Intelligence attempted to jumpstart the evolutionary process of the Kree. Through a series of events discussed below during Operation: Galactic Storm and the Destiny War, it arranged for a large number of its people to be irradiated with a Nega-Bomb (killing 90% of the Kree), and then artificially sped up their evolution by means of an artifact called the Forever Crystal.
This new brand of evolved Kree, called Ruul, have a grayish skin tone and many shoulder-length tentacles atop their heads. Individuals possess the ability to consciously 'will' adaptation of physical form to different environments, affording them controlled metamorphosis, spontaneously developing the ability to breathe underwater, fly, change form or whatever their circumstances require.
The Inhumans later brought to light a long-held secret about the creation of the Kree. As it turns out, the Kree are in fact of Universal Inhuman stock, the very first of the first of their kind. Unlike a great many races throughout the universe, the Kree were alternated by two races of cosmological abstracts in the likes of the Celestials and the Progenitors, which evolved them from primitives to empire builders. While the Celestials went on to spawn the Kree born Eternals, it was the Progenitors, a race of universal gardeners and cosmic scientists that made their home at the far end of the cosmos within the heart of a lone star, who through the introduction of the Prima Materia which the Progenitors mine and process on their World Farm called The Primagen, were able to force evolve the cro-magnon Kree race into the cosmic conquerors that they are today.
Kree - Wikipedia