Ayacho currently has the most seniority in H!P, joining the company in 2004 as a first generation Egg. Yuka didn't join until 2012/2013, so she's a bit further down the succession line.
"Leader of H!P" is mostly a ceremonial position from what I know; if Maimi was any indication we should expect to see an example of Ayacho's leadership duty at Duu's graduation. (Rio's grad should've been first, but, well...)