bossy person; self-appointed manager; bossyboots
Colloquial, mildly derogatory term for someone who takes charge without being asked, often organizing or directing others in a pushy way.
He's always a bossyboots, wanting to decide everyone's plans.
That self-appointed manager is running the meeting again.
Means 'meddler' or 'pushy person', focusing on intruding into others' affairs. 仕切り屋 emphasizes taking control and organizing, not necessarily meddling.
Compound of 仕切り (managing, partitioning) from verb 仕切る and suffix 屋 (person with a trait). Literally 'a person who manages/partitions', used for someone who bossily takes charge.