if a blood or breath player is present in a session, is one of them always destined to be the "leader"?

Not necessarily. Rufioh for example seemed to be more Detached, and I could imagine certain Breath Players could be pretty full of themselves, not quite managing to lead others. Blood is all about Bonds and Attachment, on the other side, so maybe they could be more Clingy and not wanting anyone to fight, more as a Pacifier than a Leader?