Beware some post game spoilers are mentioned below:
Mina - Literally didn't exist due to the plot-rush at the end (pretty much like the Totem Kommo-o if you had a Fairy move).
Keep in mind you can visit Mina's house to find out a bit more about her. Also she does have two notable appearances in the post game.
Ilima - Well he's the first and the normal-type one and is as generic as Trial Captains go.
Aw, you don't like his two sided rich pretty boy/enthusiastic battler depiction? I assume you did visit his family and do the extra battle against him after the first grand trial?
Team Skull - This one's talking about the grunts as a whole.... they're sort of like Gladion, but they have better external exposition that paints their tragic story better than Gladion ever recevied, so they're more empathetic than their enforcer by a couple miles. They subvert the "send kids out on adventure and succeed", showing what really happens when you fail and the world (or region) constantly ostracizes you for it. Sure, they aggravated the situation on their own past some point when they completely gave up and turned punk, but you could tell it was caused by both parties.
Do we ever see any signs it was caused by both parties though? I don't recall any. All we know is that Team Skull includes a lot of failed trial goers among their ranks, and the way this is referenced sounds more like these trainers chose to not go home or face their failure or try to do something else and decided to blame everyone else instead (you'll find in the postgame a number of people are perfectly willing to help them find work).
Plumeria - Sorry, but "Big Sister" is even more generic than Gladion, with pretty much no backstory of her own and no association to even Guzma on a more personal level. Her tidiness contrasts the Team interestingly, but that only served to make her feel like a hired secretary for the Team due to her relative distance from the rest.
Maybe you'll like her a bit more in her postgame appearances? They explain/hint at a bit more about her.
Her past is something I would like to see explored though. We get a vague reference that there was something wrong or tragic about how her journey as a trainer went, as she states when she challenges you for the champion title that she decided to start over as a trainer "the right way."
Regardless, I'll always love her heartfelt apology to Lillie and reasoning for asking that Guzma be saved. "No one can make up for what they've done if they're gone."
Plumeria states in her introduction that she keeps Team Skull together, and perhaps surrounding herself with such pitiable people gave her a feeling of being needed and wanted that she didn't have beforehand.
Lusamine - Well, I assume you know the backstory as well, so I'll skip those parts. Tragic? Yes, but that makes her a weaker villain than say... Ghetsis as a result. It's nice to have a break from "World Domination" villains, but Lusamine as a mere "puppet victim" of the first Ultra Beasts just felt underwhelming in some aspects. Even her Pokemon Team reflects that, they're just generic Pokemon with no hint of kindness (even the classic Crobat or the likes of Togekiss in her case would have helped) or non-redemption (Frustration Hydreigon is the sole reason why Ghetsis is the best villain).
You don't think there was any cleverness in having her last pokemon in every battle be Bewear, a pokemon that hurts others by trying to love them?
Lusamine was a victim, but it would have helped if it was demonstrated that she was a "notable victim" than... "just another victim" in Nihilego's tuesday. The backstory presented to us via Lillie and Gladion wasn't sufficient and jarring game-differences when we access Poke Pelago further worsens that state.
I'm not sure what you're saying here about what wasn't sufficient about the backstory, but I do think it should be noted that my impression is that Lusamine was not "just another victim" but rather was quite an extraordinary one. I doubt anyone had ever been so consumed by the venom that they went off the deep end like Lusamine.
And it should be noted Nihilego itself did not really intend for anything bad to happen to anyone, using its venom like that is simply how it protects itself and by all accounts passing through the wormholes tends to put the Ultra Beasts in rather crazed states for a time. It couldn't have known Lusamine would get addicted to the venom and obsessed with it to that extent.
But he doesn't use the name Mohn, does he? As obvious as they might take it, I don't believe Mohn is ever directly said to be the father.
Well we know from the people at the Dimensional Research lab that there was a professor named Mohn and that he was heavily associated with Ultra Space research. We likewise know Lusamine's husband made breakthroughs in studying Ultra Space. We also know people who are trapped in Ultra Space for an extended time can have their memories heavily damaged (like Anabel), hence the idea that Mohn emerged from the wormhole at a later time without knowing much aside from his name, and unfortunately didn't meet up with anyone who recognized him before he went off to live on a remote Poke Pelago island.
But yeah, its not directly said but then neither was Silver being Giovanni's son, at least for the first few games.
I mean among other things, after kidapping Lillie she just disappears from both Climatic events of the game and just asks the player's help to save Guzma in a very downplayed scene.
Do you really think its downplayed? I thought it was pretty awesome how she just casually strolls in and tells the whole group of grunts to stand down before giving a surprisingly sincere apology and explanation for what has happened.
Also, given how much she hated the whole "kidnap Lillie" plan it makes some sense that she got away from Aether as soon as she could so she wouldn't have to face what was happening.