Well, we haven´t really seen Mercury´s semblance yet (if I recall correctly), and if it was a reference to Huntik, we´d have a character referencing a show his voice actor voiced a character in.

Also, once you´ve watched Huntik, you´ll probably agree with me that Mercury would have to step up a lot to keep up with Cinder and Emerald.

I mean, what would the Fall Maiden and the future Spring Maiden want from an assassin who, as far as we know, only managed to kill his dad (who was probably so drunk he was almost dead already) and a book store owner (who looks a lot like Wolverine´s brother Sabertooth and may have similar healing powers)?