I'm counting
Mugen Senshi Valis for Duo as part of this. I know it was made later, but it is a remake of the original
Valis, which came out in 1986.
Anyway, it's not a bad game at all. A bit basic, though; it feels a bit like a mid-'80s arcade game in some respects. You get a few magical spells, something I didn't entirely learn until trying to figure out a tough boss and checking what I was doing wrong. Of course, right after that, I figured out the pattern, so... yay? You also upgrade your sword with various icons that give different shots. You can collect up to three of these, almost like a shooter.
Yuko moves pretty slow, and a lot of enemies are quite fast. The only speedy move she has in her arsenal is a sliding kick that is blisteringly quick, but also has a good amount of recovery time when coming out of it. It will kill enemies, though, although you bounce off of ones that take multiple hits. The last boss seems like a pain, but careful play in the last stage will get you outfitted pretty well for the final fight. I went with the homing shot, as it helped take out his first form. The second fell by trying to stay close and crouch-slashing. This didn't always work, but I would keep hoping the RNG would trigger his morningstar move instead of the bomb projectile, which can be a bit hard to dodge.
The other notable bit (and perhaps it was in the original as well) comes in the form of cutscenes. And boy, are there a lot of them. That death scene for a particular character (who I didn't particularly know I should care about) took a long time, in particular. Voice work litters the game as well, but given that it's in Japanese, I have no idea what they were saying. I got the overall gist of it, though. Regular school girl gets magical powers to defeat evil dude. And might have lost a brainwashed friend along the way. Probably all I needed to know!

Since I was playing on the Duo, no save states for me. Most bosses aren't tough, but the stages can leave you in bad shape without much margin for error. On several, if you die on them, chances are you'll be too underpowered to pull off the win upon respawn, so you'll end up taking a game over and go through the stage again.

That's part of why it took me the better part of two hours.
I'd say overall, the game is worthy of a 6/10. A bit above average, maybe worth a rental back in the day. One likely wouldn't revisit the game that often.
(On a complete side note, finally got some CDs that the Duo (relatively) likes. Older Sony discs made in Taiwan circa 2004 were employed on this burn, and other than missing an occasional read (and it would catch it on the next pass), it worked fine from start to finish.)