Eh? I would've expected someone to cross Youmu with Hamakaze instead of Tenryuu, but I guess because Tenryuu uses a sword?
It's the personality. Both of them talk tough and act aggressive but are easily cowed by very scary people close to them - Yuyu-sama and Tatsuta respectively.
Edit: If you don't think Youmu talks tough... see PCB and IaMP.
