Hatterene is a dual-type Psychic/Fairy Pokémon introduced in Generation VIII, revealed via leaks from various players of review copies of the game on November 3rd, 2019. It is the fully evolved form of Hatenna.Hatterene has a Gigantamax form.

Hatterene is a small humanoid Pokémon with a thin, white body. Its head is round, has a small mouth, narrow black eyes with white pupils, and two elliptical pink markings over each of its eyes resembling eyelashes or eyebrows. It has extremely long, layered, symmetrical hair, far longer than its body is tall; much like a cloak, the hair on its head drapes to the ground and swathes around it to conceal much of its true body, which is adorned by a pink semi-translucent dress that gives it a much taller silhouette, resembling that of a stereotypical sorceress or witch. This hair is colored in a gradient of baby-blue, pale pink, and white from its upper portions downwards. The hair above Hatterene's head is shaped like a wide-brimmed witch's hat with a conical shape serving as the crown of the hat formation. Extending from the pointed end of the hat is a single tentacle that ends in a pink bulb and a consecutive baby-blue bulb that has a trio of sharp claws, which can articulate in a manner similar to fingers, jutting out of it. Hatterene is a female-only species.Hatterene,Grimmsnarl,and their evolutionary families can be considered counterparts.

Both are Generation VIII Pokémon evolving once at level 32 then at level 42.
Grimmsnarl and its pre-evolved forms are male only, while Hatterene and its pre-evolved forms are female only.
Their final evolved forms are both Fairy-type Pokémon with identical base experience and catch rates.
Each Pokémon in each evolutionary line have the same base stat total as their counterpart (e.g. Grimmsnarl and Hatterene both have a base stat total of 510).
Both Pokémon also have a Gigantamax form, a signature G-Max move, and use their body hair as their primary weapon.
In addition, both Pokémon are based on mythical beings from European folklore and mythology: Grimmsnarl resembles trolls, ogres, and goblins and Hatterene resembles witches.
Lastly, both are the signature Pokémon of two of the rivals in Pokémon Sword and Shield, Marnie with Grimmsnarl and Bede with Hatterene.

Hatterene has a solitary and reclusive nature, emitting psychic powers strong enough to cause headaches as a method to deter others from approaching and disturbing it. It has also been observed that it dislikes loud noises, tearing apart perpetrators with the claws on its tentacle as a result. Due to its long hair obstructing its floating body, Hatterene's main forms of locomotion are levitation and short-distance teleportation. This Pokémon is also known as the Forest Witch.
(The above content is from the bulbapedia.)