Estelle is one of the Main Trio of the Mad Tribe. She has appeared in every episode, in some form or another.
Like all the characters, Estelle's backstory is murky. As best as can be understood, Estelle was born Clara, a poor nomadic thief living sometime when the theremin was already invented but people kinda dressed Victorianish (who knows). The Storyteller was also in her tribe. After her father died, she met Bonesy and they became friends. She eventually became a time-traveller along with Teller and Bonesy.
Estelle travelled with Bonesy and Teller for an untold amount of time. At some point she, Teller, and Bonesy became an internationally famous pop group along with Lady Bonesy. She convinced Teller to run for President of the World with his bonesy powers.
At the end of time, she and Teller convinced Bonesy to sip their souls and let them die. After her death, she met Teller at the gates of the afterlife, where he was waiting for her. They were faced with two doors-- one light and beautiful, one dark and terrifying. Although the dark one was meant for Estelle, she pushed Teller out of the way and stole his afterlife, damning him to dark torment.
Estelle grew bored of being an angel really quickly, and when a despondent Bonesy offered her a chance to regain her body, she jumped at it. She immediately plotted how to get Teller back as well, perhaps due to guilt about stealing his good afterlife. She and Bonesy went back to their own past, against the rules, to try to steal a past version of Teller that wasn't all dark-angel broody. However, she further broke the rules, and spacetime, by contacting her past self (with a giant mallet to the head). As a result of her actions, the Shadows began to overtake the multiverse.
Bonesy and Estelle convinced Teller to help them on the grounds that the universe was pretty well fucked anyway if they didn't.
The Trio entered the time stream and traveled forward to escape the Shadow Queen and try to find a weapon of Estelle's that could defeat her. In Archduchess' Love Shack, where Estelle hid the weapon, Estelle confessed her attraction to Teller. She then shortly discovered that DJ Umlatt had borrowed the weapon and had it in 2093. Before they could get it, Estelle was snatched by the Shadows. As Teller and Bonesy continued the mission to find the weapon, Estelle reached out fron the Shadow Realm and both she and Teller confessed their love.
WTF is She?
Estelle is not a Bonesy. She is kinda immortal, but just because Bonesy loves her and won't take her soul, or let another Bonesy do so (until the end of the last episode of the first season). She is basically a human, with an asterix. She does age, though time travel and other weirdness prolongs her youth. It's unknown if she's immortal now that she's been given her old body back.
Estelle is by far the most evil of the recurring characters, though not meanly so; rather she is self-absorbed and carefree. She views murder, death, destruction, and suffering as a means to an end, and is happy to be evil if it gets her what she wants. She does seem to have some ability to feel bad about past actions when they impact people she cares about.
Estelle is best friends with Bonesy and is on a somewhat equal footing with him, though she gets away with more than he'd like by virtue of being his favorite person-- even more so once he experiences her death and life without her.
Estelle is also close with the Storyteller, though she first seemed to view him as a bit of a lackey, and manipulated him constantly. She also completely screwed him over in her quest for a not horrible afterlife. Once she was brought back to life, she realized she loved him, and he loves her as well.
She has thoroughly pissed off the Angel Ramiel and probably God as well.