koibito 恋人 vs. aijin 愛人 -- so it’s only half identical. 人 has kun’yomi hito (from Old Japanese *pi₁to₂), with voicing of the initial consonant in the compound word koibito, and kan’on reading jin. If Wiktionary can be trusted, koibito is the generic term for ‘lover/boyfriend/girlfriend’, whereas aijin was borrowed (regularly) from Chinese to translate the English terms ‘lover’ and ‘sweetheart’, underwent semantic shift in Japanese, and ended up meaning ‘mistress’.
Interestingly, Chinese 愛人 àirén is just an old-fashioned word for ‘lover’, and the word for ‘partner in an extramarital relationship’ is 情人 qíngrén… except Valentine’s Day is qíngrénjié. Wiktionary also thinks there’s a difference in usage of 愛人 àirén between the PRC and the ROC, but it doesn’t describe it.
(Why was rén borrowed as jin? I’m guessing there are borrowing patterns, like how English has borrowed enough from Latin that new Latin borrowings will mangle the vowels in entirely predictable ways, but I don’t know what they are. My first guess was that kan’on readings are based on a dialect of Chinese that had the same ȵ > ȵʑ > ɻ shift as Mandarin. I figured that was too simplistic, but given that 日 has the kan’on reading jitsu and the go’on reading nichi (go’on was earlier than kan’on), it might be right. Aijin is almost certainly regular, since 刃 is rèn in Mandarin and has kan’on jin. *ɻiC > ɻəC? Could be, since the apical vowel can’t occur with a coda consonant.)
(edit: I should probably point out that I don’t actually know most of this stuff—I just know how to look it up. So my sources could be wrong or I could be misinterpreting.)
In Japanese, IIRC, one of these is ‘koibito’ and the other ‘aijin’, written with almost identical kanji, both meaning ‘love person’....
koibito 恋人 vs. aijin 愛人 -- so it’s only half identical. 人 has kun’yomi hito (from Old Japanese *pi₁to₂), with voicing of the initial consonant in the compound word koibito, and kan’on reading jin. If Wiktionary can be trusted, koibito is the generic term for ‘lover/boyfriend/girlfriend’, whereas aijin was borrowed (regularly) from Chinese to translate the English terms ‘lover’ and ‘sweetheart’, underwent semantic shift in Japanese, and ended up meaning ‘mistress’.
Interestingly, Chinese 愛人 àirén is just an old-fashioned word for ‘lover’, and the word for ‘partner in an extramarital relationship’ is 情人 qíngrén… except Valentine’s Day is qíngrénjié. Wiktionary also thinks there’s a difference in usage of 愛人 àirén between the PRC and the ROC, but it doesn’t describe it.
(Why was rén borrowed as jin? I’m guessing there are borrowing patterns, like how English has borrowed enough from Latin that new Latin borrowings will mangle the vowels in entirely predictable ways, but I don’t know what they are. My first guess was that kan’on readings are based on a dialect of Chinese that had the same ȵ > ȵʑ > ɻ shift as Mandarin. I figured that was too simplistic, but given that 日 has the kan’on reading jitsu and the go’on reading nichi (go’on was earlier than kan’on), it might be right. Aijin is almost certainly regular, since 刃 is rèn in Mandarin and has kan’on jin. *ɻiC > ɻəC? Could be, since the apical vowel can’t occur with a coda consonant.)
(edit: I should probably point out that I don’t actually know most of this stuff—I just know how to look it up. So my sources could be wrong or I could be misinterpreting.)