Mika Akitaka's MS Girls stuff on Yatate Bunko. The kind of stuff he's working on nowadays.
Download the PSP ISO for the Ginga Ojousama Densetsu Collection game here.
TV Tropes entry. Contains useful notes about the third game which are marked as spoilers. Completely pointless for a series like Yuna. Some suggestions for similar anime are also here.
Some Korean wiki page about Yuna. Jump to characters subpage. Honestly I don't know much about the Korean language. But surprisingly "Yuna" is a common female name in Korea!
A really old Geocities fansite about Yuna, beautifully preserved in its 1990s aesthetic thanks to archiving teams. There's also a few information about the characters and even Romaji lyrics!
Someone else's Yuna fanpage! There's even an attempted translation guide for the 3rd game!
Another attempted translation, but this time it's just the intro to the first game.
Yet another Yuna fanpage, but it's by huge anime fan Hitoshi Doi, and the site has lots of information on other animes as well!
My Japanese friend Ryunosuke, who has helped me with certain background information about the series' production. His site is obviously written in Japanese! He also does gameplay videos including those of Yuna on his Youtube channel and tweets occasionally. (Yes, he's one of those Japanese Twitter users who tend to take mouth-watering photos of delicious food!)
chaoskaiser72's site, where he uploaded scans of the Saturn Yuna Remix box of which he actually owns. Click here to jump to that section. Long story short, he actually thinks Yuna is failed garbage, and is a rather mean-spirited person to talk to. This is his Twitter account and he also has a Discord server.
Some of these sites also sell fascinating homebrew titles for the PCE/TG16, but honestly I'm not interesred in the consoles. I'd rather only buy the games themselves (lol).
A fascinating page dedicated to both the PCE and its successor the PC-FX
A PC Engine site made by some guy from the UK under the name Sunteam
Hardcore Gaming 101, a gaming blog. Jump to their articles on Galactic Policewoman Sapphire and Keith Courage or even buy their book on obscure Japanese video games. (It even has Tuna on the cover!!!!!!! see here)
PCE Dev - a blog
Angry Video Game Nerd's video on Darkwing Duck for the TG-16, a game adaptation of a Disney animated cartoon show (English audio with multilanguage captions)
A rather interesting page about the TG16 and Keith Courage (archived)
A blog page talking about Keith Courage and the feverish launch of the TG16