Lanie was the 17 year old daughter of the Gym Leader Norman, but to his utter dismay, she refused to train Pokemon. This was because she was rather terrified of Pokemon, after an incident in her childhood where she was attacked and severely injured by a Beedrill when the creature went crazy after being forced to Mega against its will. Unfortunately for her, shortly after moving to Hoenn, she came across Professor Birch being attacked, and begrudgingly used one of his Pokemon to save him. After touching the Pokeball however, something strange occurred and it transformed into a Great Ball, and when she released the Pokemon without, it revealed itself to be an Absol. They examined the Pokemon's trainer ID, and found that it matched the ID that had been generated for Lanie, despite her not having any Pokemon. It's name appeared to be a single symbol, and they started to refer to the creature as Black Star.
Lanie wanted nothing to do with him, and went to go and release Black Star into the wild around Littleroot Town, only to be lectured on how irresponsible it was of her to release him into an environment he'd never experienced before by Birch. He convinced her that she needed to travel to Route 120 and release him there. And thus their journey began. As time went on, Lanie found that her dislike of Black Star, and other Pokemon, went to begrudging tolerance, to enjoyment, even though she refused to admit it anyone. Some time after Steven gave her a Key Stone and an Absolite (artistic license, just go with it), Lanie found herself thinking that maybe her bond with Black Star was strong enough now that they could get Mega Evolution to work without him going berserk. She wasn't exactly sure if it would work, but decided that she wanted to give it to him anyway, to try and show him that she actually enjoyed his company, and didn't plan on releasing him into the wild when they got to Route 120, if he wanted to stay.
Shortly after making this decision, Lanie was challenged to a battle by a Ranger and his Vigoroth. Black Star attacked first using Psycho Cut, and to both of their surprise, it survived on a sliver of health. The creature roared as its trainer gave the order to use Counter, and its strong claws ripped into the Absol far deeper than anybody expected, tearing his throat out. Both trainers were at his side in a flash, trying desperately to revive him, but he was too far gone and died in Lanie's arms. So as she sat there coated in her friend's blood, a wave of regret washed through the shock, and lamented that she'd never be able to show him just how much she cared about him, and about the form he'd never be able to take on.
... Yeeaaahhh Nuzlocke's (or, in this case it was a Wonderlocke but whatever) get real painful if you get too attached to the Pokemon. Trust me, I sat there in shock for a long time after Black Star fainted. Oh the feels. Especially since I'd been thinking that I'd go get the Absolite from the Safari Zone soon and how that scene would play out in universe.