Elemental

Elementals (精霊), like Daemons and Angels, are a race of naturally forming Spiritual Lifeforms. They reside both in nature as well as the Elemental Realm and often lend their power to people they like if they ask for it.

Background
Elementals, as the name suggests, are beings who embody a specific element. They come into being from either a large gathering of natural energy or as offshoots born from a higher elemental. Their spiritual bodies are capable of taking magical energy and transforming it into natural energy in the form of their respective element which, unlike regular magic, fully obeys the laws of physics.

Evolution Line
The vast majority of the race is made up of low tier, Lesser Elementals which have knowledge and sentience but near non existent personalities. They'll lend their power to almost anyone, but in turn the power they offer is minimal.

Beyond Lesser Elementals are Medium Elementals, Lesser Elementals who have lived long enough and gathered enough natural energy to evolve into a higher species. Medium Elementals take on a proper appearance and have much more present personalities (sometimes even able to hold conversations) and can fight properly, making them a much more useful ally in battle when summoned than lesser elementals, who can only lend you their power.

Greater Elementals are the highest stage of elemental that one is likely to ever see in their lifetime. They take on a much more humanoid appearance and have firmly established personalities. The power they wield is immense and usually well within the realm of Special A, making summoning one the most reliable option to fight Arch Daemons. The caveat to this power is that they're highly fickle beings who only lend their power to people who they find both exceptional and that they like on a more personal level. Gaining the support of a Greater Elemental, much less being able to summon one, is something that only happens on rare occasions and such people tend to go down in history as legendary heroes.

At the farthest end of the elementals' normal evolutionary path are Elemental Lords, legendary beings that you're more likely to find in fairy tales rather than actual historical events. Elemental Lords are immensely strong beings comparable to Disaster-Class threats such as Demon Lords and lend their power on extraordinarily rare occasions, usually only in the form of contracts made with Spiritual Magic rather than in summoned form.

Beside their normal evolution path they can be incarcerated into vessels and become Fairies. Every species of Fairies have their own path of evolution.

At the very top of the elemental hierarchy lie the Great Holy Spirits – powerful, but egoless, elemental forces that have been around since the dawn of creation. The Elemental Queen is a unique individual who wields all eight elements and rules over all elementals.

Fairies
Fairies are elementals who have incarnated into physical form, usually by pure chance. Examples include Pixies such as Ramiris in her "child" form, and Dryads such as Treyni and her sisters.

Daemons & Angels
Although rarely talked about, both Daemons and Angels are species derived from Elementals, being Darkness and Light elementals respectively. What separates them is the nature of their existence. Daemons are almost purely magical/demonic beings while angels are almost purely holy beings, unlike regular elementals which are usually neutrally aligned (although half demonic elementals such as the corrupted Ramiris and Charys exist). This leads to a three-way dynamic where neutrally aligned Elementals are strong against Daemons, Daemons are strong against Angels, and Angels are strong against neutral Elementals.

Chosen Hero
Chosen Heroes are individuals who have been chosen by either an elemental of light or an elemental of darkness and are blessed by the Elemental Queen. They form a symbiotic relationship with their respective elemental and dedicate themselves to the betterment and protection of mankind.

Known Types
Elementals come in eight different elements, put into two different categories. The "Lesser" Elements of Space, Earth, Water, Wind, and Fire, as well as the "Greater" Elements of Light, Darkness, and Time. Although it is rare, it's possible for Elementals to gain more than one element, such as the Wind-Water Elemental Rimuru gave to Ryota Sekiguchi, or Charys who gained the Wind element from Veldora's influence.

Trivia

 * Despite having a racial advantage against daemons, they have yet to win a single fight against daemons in the series. In fact, all elemental VS daemon battles ended in the elemental's instant defeat, because they were matched against ridiculously strong Primordial daemons.