maid, servant girl, cute girl
妮 centers on a young female servant or maid, with a nuance of cuteness or charm. The meanings 'maid', 'servant girl', and 'cute girl' all describe a girl in a serving role who is also endearing.
妮 combines 女 (woman) with 尼, which likely contributes the sound. The character is used for a young female servant or maid, with the woman radical indicating the female role.
A woman 女 beside 尼, which looks like a person sitting under a roof, suggests a maid or servant girl who works indoors. The shape also feels delicate, hinting at a cute girl.
For ジ, imagine a cute maid named Gigi who serves tea: Gigi -> ジ, and her name matches the reading.