2 John reads like I John in Cliff Notes: love one another, to love God is to obey Him, Jesus has come in the flesh, beware anti-christs who "run ahead" of the word of God and would lead others astray. The letter's brevity is explained: some things are better said in person than in writing, I hope to visit soon.
The most intriguing part of John's second canon letter is his greeting, "to the chosen lady and her children," and his closing salutation, "the children of your chosen sister send their greetings."
Who is this mystery woman?
She is NOT the "whore of Babylon", the adulterous femme fatale of the Book of Revelation. She is a chosen lady, held in high esteem.
The most obvious answer is that "she" is a local church, a local expression of the Bride of Christ. Other thoughts and possibilities:
She is a particular but unnamed woman, a Christian of note, like Dorcas and Priscilla and other faithful servants of the Master mentioned in the Book of Acts.
She is Mary, the mother of Jesus. We know that even as he died in agony, Jesus charged John with Mary's welfare: "son, behold your mother." Strong early tradition associates both Mary and John with the church of Ephesus.
Returning to the first idea - a specific local church - one might ask "which one?" Again my wild hypothetical guess would be Ephesus, and not only because of John's presumed association with that major Roman city. Ephesus already had a strong goddess cult ("Great is Diana of the Ephesians!" the mob chanted as they nearly killed Paul). It is possible that the Christians, in counterpoint to this well-funded, entrenched, aggressive cult, publicly assumed their legitimate identity of the Bride of Christ. "We do not worship a false goddess; rather we are the bride of the risen, living Bridegroom."