Ryudia is potentially the oldest of the Yuna characters; one of the files I have lists her birthday as June 3, 2 B.C. One way or the other, she was indeed the former Savior of Light before Yuna was. I'm not really clear on why she isn't anymore, but when you run into her in the first Yuna game she's one of the Army of the 13 Frauleins of Darkness. For millennia she's been hidden (imprisoned?) in an undersea temple on a tropical oceanic planet. A huge statue of her sits on the beach, labeled "The Purest Warrior Ever" or something like that. As part of Yuna's mission in the first game, she must gain access to the shrine and defeat Ryudia -- but she doesn't know that until she's already face-fo-face with Ryudia herself! When fighting Ryudia, Yuna isn't allowed to use the extra armor/weaponry provided by Jina, Erina and Marina; this suggests to me that perhaps the Savior of Light has to defeat the former holder of the position one-on-one to truly claim the title.

Before Yuna defeats her, Ryudia is wearing a suit of golden armor and wielding a golden sword. However, when she shows up a bit later she's wearing the flowing sigiled robes pictured above. It appears she's normally a sorceress; she has a couple acolytes in wizard outfits assisting her and she casts some cool spell with a big rune on an altar to make the portal to the World of Darkness open up so Yuna can go face off with the Queen of Darkness. Also she has a telepathic link with Yuna since Yuna and she have both been the Savior of Light. They only seem to be able to communicate at certain times, though; probably has to do with Yuna's strength of will or something.

In the second Anime OAV (but possibly only in the dub), somebody says Ryudia has "the most powerful fleet in the galaxy." In the third game and the first OAV she shows up in a big gold and red cone-shaped starship. In the first OAV, when Fraulein D's huge fleet of ships comes out of warp, a minute later a whole bunch of similar red and gold ships come out of warp to fight them. I'm assuming that's Ryudia's fleet. The details on that are probably revealed somewhere in the part of Yuna 2 that I haven't played yet.