The senate has, in theory, the same power to pass/change/remove laws as the Emperor/Empress does, although the monarch has more power because his/her decrees go into automatically and instantly, while the senate votes on things. Unless there's some significant urgency for a law, the senate usually handles legal stuff. In the Yamataian government, there is no need for an Emperor/Empress, so if one resigns, dies, or is kicked out for incompetency (in theory, the Ketsurui clan or the Senate could), the Senate could rule for years without picking a new one (such as the two years after Empress Yuumi died).