Valeena

Valeena is an antagonist from animated series Super Robot Monkey Team Hyperforce Go!, first appearing in the season three opener but primarily appearing in the fourth and final season. Despite her presence in the show not actually being that apparent (only six episodes, one of which she barely shows up in at all, and the last two are the two-part season four finale finale), she's remembered and well-liked among the fandom for her role in the plot of the show, her distinctive design, and delightfully evil personality.

Basic Information
Valeena was born to Ma and Pa Shinko, two seemingly ordinary and innocent background characters who are revealed in Season 4 to have been leaders of a cult worshipping the main antagonist of the show, Skeleton King. Valeena was offered to Skeleton King as a disciple, and she was given some of his power before being cast to the Savage Lands, where she would wait for who knows how many years before the monkey team showed up. When they did, she captured them and held them at bay while she attempted to sacrifice Jinmay, robot girl and love interest to main protagonist Chiro, to the Skeleton King, who was. Kinda merged with a giant eldritch worm at the time. Don't question it.

At the end of the day, she was knocked into a Formless pool and seemingly drowned in there, never to be seen again, only to make a reappearance in the season 4 episode Ghosts of Shuggazoom. After that, she became a recurring antagonist throughout season 4.

Burton
I mean, you've got the dress, the pigtails, the spines and metallic neck, the black and gray color scheme...she really has the aesthetic nailed down.

Angst
Let me reiterate - her own parents offered her up to an undead being serving eldritch forces, had her get turned into something similar, and then dumped her into a jungle where, among other things, there are dinosaurs running around, effectively robbing her of a childhood and a normal life and giving her only Skeleton King to turn to. Then, after the entire season where she works so hard to bring him back from the dead, fight off his enemies, and succeeds at doing both, what's her long-awaited reward? Getting blasted into ashes by her own master. Needless to say, there's a reason people say Valeena deserved better.

Power
After she came back from the dead, she had a few months and a girl who can turn into a giant robot (who she also attempted to sacrifice to an eldritch worm) as her main opposition. She managed to do what even Skeleton King could not and actually take over Shuggazoom, turning most of the population into ghost-like Wraiths by the time the Hyperforce came back from destroying the Skeleton King Worm.

Dominating
I could cite several things for this, but instead I'm going to link this clip because it does a better job making the argument for this trope than words ever will.

Twisted Freaking Cycle Path
Being raised in a cult does not do wonders for your sanity and morality, which can also be seen in the clip above.

Girlboss
"Did Valeena effectively use girl power when-" Yes. Next question.

Villain
If it wasn't obvious enough by the page so far.

Monster Features
Was a normal human at one point, but the Skeleton King's power transformed her, giving her pointed ears, gray skin, a nearly nonexistent nose, and a metallic neck with spines.

Eldritch
The Skeleton King's power comes from eldritch abominations, and some of that leaks over to Valeena, mainly in the form of body horror. She summons a horde of minions from her mouth at one point, can extend that metallic neck at will, and can extend her fingers to act as restraints.

Distinctive Voice
Shares a voice actor with Teen Titans' Starfire, and has the same sort of "otherworldy" effect to her voice, for a lack of a better word. Needless to say, it's very effective.

Purple
Her magic and her eyes are both purple.

Animal Theming
A somewhat minor one with bats, considering her go-to minion summon is bat skulls and she kinda looks like a vampire.