Lover
Mistress
noun
The person you make love to is your lover. Woooh! Don't worry I won't tell. Have fun and be safe, kids.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
コウイチの愛人は、少し鼻にかかったみ力的な声をしていました。
Koichi's lover had an attractive, slightly nasal voice.
お前は愛人にすらなれなかったんだよ。
You couldn’t even be his lover!
愛人の是非については、判断しかねますね。
I'm ambivalent about whether a mistress is acceptable or not.