Lusamine is easily one of the most tragic, if not the most tragic pokemon character. Make no mistake, the things she did were horrible and she will have to take responsibility to make amends. The verbal abuse she directed towards Lillie I never thought I would see in a pokémon game.
But wow, Lusamine's circumstances were really horrible.
In the past
She inherited a large science foundation from her father, sharing his love and desire to protect weak and otherwise endangered pokemon.
Mohn, who was Lusamine's husband, worked on a side project about Ultra Wormholes. Very different from their typical ecological research, but necessary to learn about the Ultra Beasts, which continued to threathen both humans and pokemon.
Disaster strikes
However, one day one of Mohn's experiments went wrong and he was sucked into a wormhole. He left nothing behind except his research notes and a Cosmog.
Lusamine was devastated. Upset with grief, she took over his research, determined to found out what happened to her husband. She would never be the same after this, but at this point she was a far cry from what she would turn into. It was presumably at the same time as Aether started working on creating Type: Null, for her to take "revenge" on the Ultra Beasts.
However, as her research continued, she came in contact more and more times with Nihilego. This was where the downward spiral really started to happen.
Nihilego, not even once
Before we continue, we have to talk about Nihilego. From what we know about Nihilego and its neurotoxin;
1. It makes you feel amazing and like you can do anything.
2. It is probably highly addictive, largely because of point 1.
3. It removes your inhibitions and affects your behaviour and personality.
Point 3 is very dangerous, because removing your inhibitions means that you are removing your rationality.
Someone had this as an example: what would happen to a stereotypical office worker whose greatest desire is to get promoted, if influenced by Nihilego?
First he would get highly productive, at an unnatural level. His body probably would get burned out but the toxin makes him feel fine. Due to the parasitic nature of Nihilego, the reason why he wanted the promotion in the first place will become warped and turn into something that would benefit Nihilego instead. But what would happen if he doesn't get promoted anyway? His dampened inhibitions would lead to him to act upon the irrational thoughts that would "help" him to get promoted. Such as sabotaging for his co-workers or boss, possibly even going so far to outright killing them. Because at that point, any action that will get him promoted somehow will seem sound.
Lusamine's down-ward spiral
And this is what happened to Lusamine. Her research focused less and less on finding her husband, and more on the Ultra Beasts themselves. I wonder if she maybe at some point thought that they might help her?
Her ideals of beauty and ambition to protect and preserve pokemon became warped. She started to cryonically freeze the pokemon she loved and cared about so much. That way their "beauty" would never be tarnished and be "safe" forever! Clearly!
Similarly, her kids started to become a nuisance to her. They were a distraction in her attempts to connect with the Ultra Beasts. They couldn't help her, thus were not important and "ugly". Lusamine presumably even ordered Lillie to dress up to look like the likeness of Nihilego, to be able to stand her.
This eventually turn into an outright rejection of her kids, feeling betrayed when they run away with Cosmog and Type: Null respectively.
When you meet her first time in-game, she is just a shell of her old self. She still loves all pokemon (but *just more so* if they're ultra beasts). But it doesn't take long until you notice the cracks in the facade.
When you meet her in the Ultra Space, she is at the end of the disaster spiral. At this point, the toxins have clouded her mind so much that only thing she cares about is Nihilego. She has finally entered Ultra Space, but the reason she wanted reach it in the first place is all gone. Mohn, her kids, Pokémon, even the world she came from.. none of that matter anymore to her, only Nihilego. When you and Lillie confront her the final time, she hits rock-bottom by merging with Nihilego. At this point, she is not even a person anymore, she is more or less a part of Nihilego.
This would probably been the end for Lusamine, if she had not been saved by Nebby (and suffer through probably the most rough start of a de-tox treatment ever).
The aftermath
After this point, what awaits her is the long and painful recovery from the toxins along with slowly realizing all the things she has done (involuntarily) the last couple years of her life. All things she cared about before, along with her own self, suffered because of her own actions. She had turned the Aether Foundation into something she and her father would never have allowed to, hired Team Skull to actively hurt and abuse Pokémon, she disowned and abused her beloved kids, forcibly froze her pokémon, tortured Cosmog for her own end.
What I find extra tragic is the fact is that the Nihilego didn't affect Lusamine out of any malicious intent. In fact, they seem mostly unaware of their neurotoxin and what it does. Unlike many other parasites, it doesn't infect hosts because of personal gain (like sustenance) but as a self-defense mechanism. It is just content to gain extra protection and care from something or someone.
So all this happened in Sun/Moon not because of some evil being. It was just a desperate and hurt Lusamine encountering confused and afraid Nihilego, acting on self-defense instinct. And neither of them were aware what the effects would be.
Tl;dr
Lusamine was once a happy and loving woman, loses her husband in freak wormhole accident. Desperate, she takes over his research. During this she encounters Nihilego, gets intoxicated with its highly dangerous neurotoxin and begins to change in personality. Becomes increasingly obsessed with Ultra beasts and in the process everything she cared for suffers because of her actions. Eventually, after entering Ultra Space and losing so much of her own self, she only cares about Nihilego and decides to merge with one.
Lillie, Main Character and Nebby intervene, with Nebby forcibly blasting Lusamine loose. After having most of the neurotoxins removed, she has to deal with the slow realiazations of all the horrible she thing has done while still trying to recover from the toxins.
And she doesn't even know that Mohn is still alive.
