So, I can't recall the specifics either, but there's five main characters involved here: Saya, Miki, Yuka, Nagisa, and the boyfriend (Kousuke). It doesn't help that they're referred to as a mixture of their first/last names, but:
The extreme tl;dr version: Hero Rei manipulated Saya+Yuka into the current situation, because she wants to consolidate powers a bit and because she was bored. Yuka's motivation is that she has a grudge against Miki and wants to take everything(including Saya) from her. Saya has no great motivation here and is just a victim. So all this drama is just Rei manipulating people for fun and the fallout of it.
The longer version:
Saya: This is the swordswoman with long black hair. She's the bodyguard+friend of Miki, through some form of allegiance to Miki's family line
Miki: She's just a member of some (at least semi)-powerful family on Earth. The girl with short blonde hair #1. She's friends with Saya, since they've been together forever or whatever.
Yuka: She's a branch member of Miki's family or something. Basically, she's just jealous of Miki for "having everything" and wants to ruin everything for her. She has short blonde hair (#2) and has the plate armor.
Nagisa: Short black hair, who Saya's fighting in this chapter. A good fighter but otherwise not relevant, she just gets involved here because she's good at fighting.
boyfriend/Kousuke: I forget if this guy was actually anyone's boyfriend, but he+Miki had some form of romantic connection, even if unconfessed or whatever. But he's dead now so whatever
Rei approached Yuka and gave her a plan to ruin everything for Miki, like she wanted. This involved Yuka getting intimate with Kousuke, and then manipulating Kousuke into attacking Miki (saying she was plotting against her or something, idr). Kousuke attacks Miki, and Saya responds by killing Kousuke (since she's Miki's bodyguard). This then puts a rift between Saya + Miki, since Miki had some attachment to Kousuke before he attacked her / before Yuka took him to manipulate him.
Saya feels bad about it and then when she's busy feeling bad, Rei goes to her too to basically convince her that her power would be better off with someone more skilled, which in this case is Nagisa. So Saya goes to fight Nagisa (what's happening in this chap) basically as a way to throw her life away (because her only meaning was as Miki's bodyguard, which is gone now that Miki's avoiding her) + give Nagisa the sword saint skill, since Nagisa would make better use of it. However, with the whole thing being set up by Rei+Yuka, the real point of it is just to be a show, for Yuka to swoop in mid-fight and take the sword saint skill from Saya (as a way to further take everything from Miki, killing her friend/bodyguard + taking the skill). Yuka tries this, but then Nagisa butts in and stops Yuka, Yuka gets restrained and Miki+Saya make up a bit and it's a happy ending until Saya takes a mystery sword to the chest.
Admittedly I forgot a decent part of this too, so I wrote this up as a way to remember myself since I was also getting a bit lost, lmao.
