Schuldig is one of the other Schwarz members. He's a telepath, flamboyant, sociopathic, and the name's ironic because I get the impression he is incapable of feeling or understanding guilt. He's probably Western fandom's overall favorite so the impression is understandable! But canon is actually mostly about Weiß and the Schwarz characters are just recurring antagonists. I think.

WK does a lot of that Anime Catholicism where it's really clear they enjoy the aesthetic but don't really come from a deeply Christianized culture. I enjoy the crap about of it, tbh, because I like getting a taste of our own medicine and also it's just fun, but ymmv. The Kreuz in the title is part of that. I'm not really sure it's that relevant otherwise though! I think it's just like someone throwing "zen" into the title of a Western thing for the cool value.

Schuldig is German and I think some of the rest of Schwarz's angst backstory is linked to (fantasy) Germany but tbh I'm not really sure why all the bad German otherwise, except the cool factor, I don't think Weiß actually has any connection to Germany.

I think the title is supposed to be more like "Weiẞ's cross" or "Weiß and the cross" than "the white cross" but idk if the makes the grammar better.