This is getting interesting. And the german hero beeing Siegfried makes a lot of sense, too, as he's about the only one universally accepted that actually has all that is required for a hero.
As for how Brynhildr/Brunhilde will come into play, well, in the Nibelungenlied she was the queen of Iceland and rumoured to be stronger than any man. But the king of Burgund, Gunther, wanted to marry her so bad that he asked his friend Siegfried (who, if I remember it right, possessed a ring of invisibility and a belt of change shaping) to help him win Brunhilde over. Since Siegfried himself wanted to marry Kriemhild, the sister of Gunther, despite not being royalty, he agreed and, either disguised as Gunther or invisibly helping him fight, they defeated Brunhilde three times. Not with her starting to suspect something was wrong when she dominated Gunther in bed while he easily forced her down in a brawl before. Suffice to say that she laid her eyes on Siegfried herself, who is portrayed as a handsome, muscular guy of great intellect who himself is virtually invincible.
In the end, though, she had to bow down and went with them to Burgund and at the side of Gunther became the queen of this country. Like everyone else in that story, she met her end in the most gruesome fashion after Kriemhild went crazy over Siegfrieds death. It doesn't have to do much with Ragnarok, though, as it's mostly a tale about people getting all crazy over love and going to huge lengths to either get it or take revenge on those who caused them to loose it.
So, given the connections between this manga and the tale, I agree that it's possible for Ryouta to be an image of Siegfried, what with him helping the girls so that he can win Neko over. But what doesn't fit - at all - is him being the last one standing over a dying Kuroha. Because, the death of Siegfried actually starts the downfall of everything. And while Kogorou might fit for Hagen von Tronje, it's still too early to give him that dastardly role. Among the girls, I think that Kazumi and Kuroha both share traits of Brunhilde and Kriemhild. Kuroha has the inhuman strength of Brunhilde, but the jealousy of Kriemhilde, while Kazumi has Brunhildes growing love for Siegfried and Kriemhildes intelligence.
Granted, there is way too little information on how firm Okamoto is going to base this on the original tale, so everything above regarding the association of the different characters may just be a crazy theory.
On another note, that new scientist looks interesting and may be the much needed ally Ryouta looks for within the organization. Although I found it quite funny for the driver to call her looks homely