Ekaitz is badass. He is small and petite and nearly androgenous and he still kicks serious butt. <3

The reasons for Sigma's reluctance to assume command are twofold. Reason #1 was explored mostly in Breaking Protocol (it just spills over to CtIP) and Reason #2 is Tau Ceti. Being only a Second gives him much more freedom and doesn't tie him to a ship in case Tau decides he wants to go somewhere else.