You OUGHT TO be mad that Nihongo puts girls in second place.
Honestly, unless you're specializing in contract law, you won't need this ONyomi, but it was so obvious it practically wrote itself.
As for how to remember the APPEARANCE, well, it kind of looks like the numeral '2'!
second rank, second thing in a series - usually seen on legal documents: "Party of the second part" or whatever.
pure, virginal girl
a girl. Unlike the related term 少女, 乙女 emphasizes the "pure, virginal" aspect of girlhood - used about girls around 12-19 years old.
girl, woman, female
女性 女の人 嬢 乙女 女の子 女子