WHAT YOU NEED TO KNOW ABOUT UNDERVEIL
The link above is a compiled summary regarding terms and individuals you will see mentioned on this blog (compiled by my good friend @thebonezone66 !). It's not an entire run down of the story (as I think that's still being worked on), but here you'll have a basic understanding of things such as the Garden Wall.
Lucida's place in the Garden Wall was among the Anomalies, given she is not "canon" to Undertale at all, and thus was sucked into this empty timeline on accident. The Troubleshooters were "admins" who oversaw the timeline and its happenings. Angelo(@seraphofretribution ) wasn't one of them, but interacted with them often and made visits to the Anomalies as well.
In Lucida's time there, she has been faced with highly dangerous SEEDs, has met face to face with a genocidal Chara dubbed MC (and was the sole survivor of the encounter among two others who were with her. She learned very quickly about the game of "Tag"), and has had to endure timeline resets which were extremely jarring. She has met The Dragon, but only once. Her opinion of him is not high, given how vague he was about the entire situation. There was something else going on, and he refrained from ever telling the Anomalies all the details. All finer details remained in the circle of the Troubleshooters (though Angelo was left out of said details).
In the end, the Garden Wall collapsed, and everyone was sent home. Only, when Lucida was sent home, her childhood companion Ploobie was lost mid transportaion. She hasn't the slightest clue where he is. No-one does.
Lucida is now burdened with the knowledge of her world being so much smaller than she knew it to be, knowledge her home timeline residents wouldn't ever understand, those emotions souring given she didn't have the contact of many of those anomalies she'd met to speak to.
She's been traumatized by multiple near death experiences and timeline resets, and is now on a long quest to find out what the hell happened to Ploobie. Her anger rests on those who created the Garden Wall, loosely pointed at The Dragon, and anyone else who admits to have had anything to do with it.
It's been a long road, and now she's escaping her confines to new horizons.