Tafsiran Bilangan 25:15: Rashi, Or HaChaim, Rabbeinu Bahya & Siftei Chakhamim

ושם 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).

המדינית, The Midianite, etc. The reason the Torah describes Kosbi with the letter ה for a definitive article is proof that she must have been the only Midianite woman who sacrificed her self-respect in order to seduce an Israelite. This may also account for the fact that in 25,6 the word המדינית is written with the letter ה at the beginning although we had not heard of that woman previously. In fact, if the Midianite women had participated in mass seduction there can be no doubt that G'd would have commanded that they be killed during the punitive campaign against Midian. The Torah only reported the people as debasing themselves by sleeping with the Moabite women (25,1). There was a good reason why the Midianite women did not do what the Moabite women did. It was not that they were morally superior, but the Moabites had been given an assurance that the Israelites would not touch them. No such assurance had been given to the people of Midian. The reason that Kosbi was different was that she was Balak's daughter and as such had a personal interest to do what she could to help her father achieve the downfall of the Israelites.

כזבי בת צור, “Cozbi daughter of Tzur.” She was the daughter of a king, seeing that Tzur was one of the five Midianite kings killed by the Israelites in the punitive expedition (Numbers 31,8). The Midianites harbored so much hatred against the Israelites that one of their kings demeaned his own daughter to the status of a whore in order to lure the Israelites into sin (Tanchuma Pinchas 2).

ראש 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).

בית 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.