Sanguine's first secretary/assistant was Eyjafjalla. She was one of his patients he had really close bond with, so he wanted to keep an eye on her condition throughout the day. She was also one of the people who were willing to not judge him based on his actions in Babel. Though she also proved herself to be very capable in this role
His second and current assistant is obviously Skadi and is a lot more complex to explain. Long story short, after a bit of a turbulent start, they slowly started trusting each other more and more, in large part due to them often sharing a drink at the bar and somewhere in the middle of it all he asked her to become his bodyguard/assistant. After some hesitation from our favourite bounty hunter (and frankly quite a bit of convincing), Skadi eventually agreed. And the rest is history, as we all know what happened with time~