I think it's pretty obvious that Seig and Hilde are a couple. They're both wearing the gryphon armor and are shown together in that one b/w sketch. And really, what else would they do with a girl knight other than have her be a romantic interest for Seigfried? And why would she change her family's wolf-themed armor for anything unless it was to match her husband?
I'm calling it. They're together for sure.