Mio is a humanoid offshoot of Queen Beryl from what I remember (my mind is gunna be refreshed soon enough as I am getting through the series again). Being as off shoot of Beryl, she has ALL THE REASONS to pick on Usagi the way she does. She is definitely there for a reason. Beryl is trying to torment and weaken Usagi emotionally and by creating Mio, someone who can live in the normal world, she can get this done, while having her generals do their thing.

Mio adds a lot more drama and tension to the story (something this version of SM thrives on), especially when Mamoru is captured/goes willingly to the Dark Kingdom. What gets me the most is how scared she is right before she is...off'd. It's a really great moment and you can see that Usagi wants to try to help her. Its moving because of everything, Usagi is so very forgiving and its helps to contrast Princess Serenity too. You kinda thought maybe JUST maybe deep down inside (and i mean DEEEEP), there was maybe a little bit of good in her, probably left over from before Beryl's corruption. Oh man now i'm getting excited about eventually getting to that part of the series again!

She's Beryl's shadow according to Zoisite so yeah you're totally right...

And we see her being superposed with Beryl at one point as well.
The only thing I didn't really and went 'WHY' about Mio was when she transformed in that gigantic insect monster xD

I mean... that was weird. Even Sailor Moon wise.

She was actually a giant plant monster IIRC. As far as that goes...yeah it was weird and still haven't quite linked why they chose that other than "LOL THIS IS LIKE SUPER SENTAI GOTTA HAVE A WEIRD MONSTER DESIIGN". Though, plants do show up as a small motif here and there in Sailor Moon (90s anime Tuxedo Mask's roses, the Sailor Moon R movie, the Makaijuu Arc...etc so maybe it was a nod to that??)

It looked like like a praying mantis merged with carnivorous plants. I had no idea what it was nor why it was added. I mean sure, there's the plant thing going on in most of the seasons, but that's a kind of monster we don't really see. We've seen weird monster of the day, but that topped everything. Maybe it was the point? Going over the top in order to do like the mega monster of the Sailor Moon series?

It was something I really wasn't expecting nor seen coming, so I guess it gives a bonus point despite being totally random?

She is a conniving, manipulative, annoying, and outright stupid little jerk-butt who is not only nothing but a walking queen bee stereotype with absolutely zero character to her other than being evil and conniving, but she just doesn't do anything all throughout the show! All she does is brainwash everyone into taking her side on everything, making the girls out to be outcasts when things don't go her way, sit around, and be seductive to Mamoru. She's basically just thrown in there to make things worse for the two of them, and even when she's faking being nice, she's still extremely shallow, with her "we're classmates so we're automatically friends" shtick and for guilt-tripping people if they don't so much as even look at her. Uhh...FRIENDSHIPS DO NOT WORK LIKE THAT! What is with Japan and their belief in this concept?! Friendship isn't when two people meet and boom, they're friends! Friendship takes hard work, open communication, mutual interests, compassion, and willingness to compromise and put grievances aside. It can be messy, and it's not easy, but it's always worth fighting for. This is especially jarring because earlier episodes showed the girls doing nothing but communicate, work toward their friendship, bond over mutual interests, and help each other! By throwing Mio into the mix and by having Usagi say "we're classmates so we're friends" when comforting her, they're basically going against everything the previous episodes conveyed with their messages of friendship. I know people say the show would be better if Sailor Luna had been cut out, but honestly, PGSM would be better if Mio was cut out! At least Sailor Luna actually did something and wasn't just thrown in to be nothing but eye candy or to make things worse for the team! Mio's just a useless villain for the sake of being a useless villain. There's literally nothing else to her. I was happy when a Metalia brainwashed Endymion killed her off near the end of the series. I even cheered!

Also, the plant monster thing occurs in the Special Act after being resurrected... Maybe it had to do with however she was brought back? Would have to watch it again... Definitely a necessary tokusatsu element though!! Reminds me of Germatoid from anime S season too haha.

I really like that she is the only character to truly exploit Usagi's goodness as a person and not the "I'm going to harm your loved ones Sailor Moon!!! Surrender now or they will suffer!" mustache twirling sort of way.
No matter how much she schemes and tries to humiliate or isolate Usagi, Usagi is too good to see through it and forgives her each time. Mio even gets Usagi to *apologize* for things Mio concocted! I thought her introduction and initial plot points were a breath a fresh air as far as plots go in Sailor Moon across all forms, but I feel like they decided not to have her killed or re-absorbed by Beryl so she was just kind of there until Metalia-possessed Endymion offed her.

So I think they could've gotten MORE use out of her as a different kind of antagonist than they did. I haven't watched the Special Act yet though, so I'll have to wait and see what they do with her there.

I'm sad they didn't wish to keep their powers after defeating Mio. I feel like it would fit in better with the rest of the Sailor Moon canon, as in they missed the powers and decided to accept that part of themselves to keep on the back burner as a just in case, especially since they're all separate.