Arella is a denizen of the extradimensional realm of Azarath and the mother of the Teen Titan Raven.
History
Arella unwittingly became the bride of the evil inter-dimensional being known as Trigon. Through this union, Arella gave birth to Raven and raised her in the peaceful realm of Azarath. Despite Arella's efforts, her daughter was bound to Trigon by a prophecy which would entitle the subjugation and destruction of all life. Arella forever gave Raven her love, even knowing what she was destined to become: A vessel for Trigon's evil.
When the prophecy was about to come to pass, Raven desperately sought out Azarath for help, only to find the city at first apparently deserted. A white dove led her to her mother, who could, however, offer neither help nor comfort since the prophecy was inevitable. Right afterwards, it was revealed that the sound state of Azarath had been in fact a cruel illusion: The city was destroyed and in flames, and its people - including Arella - had apparently perished.
In the DC comics, Arella was born in Gothem City, She was a depressed, aimless teenager who fell in with occultists who goaded her to participate in a secret ceremony to appeal to Trigon, who was to manifest in the earth-plane. Although she knew Trigon was a demon by origin, she thought Trigon's human form was authentic. When she realized Trigon's true form, it was already too late. After that, Arella attempted to kill herself with pills in an abandoned alleyway. Instead of dying, however, she was taken to the Azarath, located between all dimensions. It was there that she was taught pacifism and had her named changed to Arella.
Teen Titans Go!
Azarath and its people were restored along with Earth when Trigon was defeated. In issue #44 of the Teen Titans Go! comic book series, Raven is seen traveling back to Azarath once again, with the entire city fully restored. Raven makes a point of visiting Arella to discuss with her why she is now free to feel.
Personality
Arella is a very peaceful, motherly woman, but also rather fatalistic like her daughter, mainly due to the part she had unwittingly played in Trigon's scheme. Arella's name means Messenger Angel to the Azarathians.
Powers and abilites
Arella seems to be able to wield some sorcerous abilities, enough to leave a projection of herself for her daughter even after Trigon had devastated Azarath. It was then revealed that Azarath had already been destroyed, and that Arella may be dead. Although this is debatable, she must had given Raven her last words through a telepathic voice-mail of some sort. The true extent of her powers are unknown.