Rashi on Numbers 25:15/רש"י על במדבר כ״ה:ט״ו

1ושם האשה המכה וגו׳ AND THE NAME OF THE [MIDIANITISH] WOMAN WHO WAS SMITTEN etc. — This is stated to show you the hatred the Midianites bore to Israel — that they abandoned even a princess to prostitution in order to make Israel sin (Midrash Tanchuma, Pinchas 2).

2ראש אמות [HE WAS] THE HEAD OF THE PEOPLE — He was one of the five kings of Midian, Evi and Rekem and Zur etc. (31:8). He, however, was the most important of all of them, as it is said, “the head of the people”, but because he made himself despicable by abandoning his daughter to prostitution it enumerates him not first but only third (Midrash Tanchuma, Pinchas 2).

3בית אב — Midian had five fathers’ houses: Ephah and Epher, and Hanoch, and Abida and Eldoah (Genesis 25:4) — and this man was king of one of them.