mutual term of address
婭 centers on a mutual term of address, likely used between relatives or close acquaintances to refer to each other in a reciprocal way.
婭 combines 女 (woman) with 亞, which likely contributes a phonetic role. The exact historical development is uncertain, but the character is associated with a reciprocal form of address, possibly between women or in a familial context.
A woman 女 using a mutual term of address with someone represented by 亞, like a reciprocal nickname between close friends.
For ア, imagine two women addressing each other with a friendly 'Ah!' as a mutual greeting: 'Ah!' -> ア.