'Rashi': According to the simple meaning, she was a Yisraelis. Radak #1 - she is called Acheres, because it mentioned ¹ his first wife.

'Rashi' citing Yerushalmi Yevamos 2:3: He married a Nochris, to aggrandize himself. She was beautiful, and a Saris. Radak - she is called Atarah, for he married her Lehis'ater (crown or aggrandize) himself - she was the daughter of great people. Malbim - "Acheres" teaches that she was a Nochris, for it did not mention his first wife.

Radak #2: She was a Yisraelis. She is called Acheres, because she was a Pilegesh - "Ben Ishah Acheres Atah ² ."

'Rashi': Em is like "Kamti Em b'Yisrael." Onam is lowly and weak, an expression of Onan, like "Lo Achalti b'Oni" and "k'Mis'onenim." From him came Sheshan (28-31), who gave his daughter to his Egyptian slave. It discusses their detriment - they were not proper for kingship! From him came Elishama (35-41), whose grandson Yishmael killed Gedalyah. (Radak - Yishmael was the 24th generation from Atarah. From here Chazal said, do not trust a convert for 24 generations ¹ . Malbim - it says that Yishmael was "mi'Zera ha'Meluchah ² " (Yirmeyah 41:1), i.e. from his mother; he was not mi'Bnei ha'Meluchah.