I teach on spiritual friendship and, for the longest time, I have defined spiritual friendship thus:

Spiritual friends are friends who are friends with each other because of their common friendship with Christ and who are committed to helping each other follow Christ.

This definition was based on C. S. Lewis’s understanding of friendship. Lewis says that friendship starts,

when two or more . . . companions discover that they have in common some insight or interest or even taste which the others do not share . . . . (1)

For spiritual friends, their commonality is their relationship with Jesus. Spiritual friends are friends of Jesus. Lewis also says that friends are committed to common activities. Spiritual friends are committed to helping each other follow Christ.
I think my original definition of spiritual friendship still holds, but, recently, I have been thinking of expanding it a bit to include mutuality and intimacy. I got the idea from James Calvin Davis’s book Forbearance. He gives the following definition for friendship:

Friendship is a relationship of mutuality and intimacy rooted in shared interests, love, or goals and characterized by genuine interest in the other person as a particular other. (2)

I have decided to expand my original definition of spiritual friendship a little.

Spiritual friendship is a relationship of mutuality and intimacy rooted in a common friendship with Christ and characterised by a commitment to help the other follow Christ.

I have incorporated Davis’s concern for mutuality and intimacy.
One of the characteristics of friendship is mutuality. There is a give and take in friendship that is not integral to other relationships. You may ask, “How does mutuality work if one friend is older, wiser, and more accomplished than the other?”. I think back to some of the professors that I had in Regent College. They were older, more educated (one had two doctorates), more experienced in life and ministry. But they invited me out for meals and seriously invited my input on matters of common interest. Coming from Asia where the relationship between teacher and student is much more formal, I was surprised at first but came to appreciate the friendship offered to me by my teachers. I never forgot they were much more mature than me in so many areas and I always respected them, but they did offer friendship, and many remain friends to this day though some have graduated to heaven.
Jesus, truly God and truly human, calls His disciples friends (John 15:15). Was there any mutuality in their relationships? I think of Jesus asking Peter, James, and John to be with Him in the Garden of Gethsemane (Matthew 26:37). When you are facing a major challenge in your life, you want your friends with you. Jesus entrusts the care of His mother to a close friend (John 19: 26–27). And Jesus receives help and support from a group of lady friends (Luke 8: 1–3). Jesus is Lord and God. He is also a friend. And as is often the case, it is the more powerful person in the power differential who initiates the friendship. But, yes, friendships are mutual even if one of the friends may have more to offer.
What about intimacy? We don’t need to look any further than Jesus’s relationship with His disciples. He knew what was in their hearts (John 2:25), He washed their feet (John 13:1­–17), He loved them to the end (John 13:1). And above all, he gave His life for them (John 15:13). The Old Testament go-to for what a friendship can and should be is the friendship between David and Jonathan (1 Samuel 19:1–20:42). Here is a commitment to a deep intimate friendship (1 Samuel 20:16–17). We often associate vows of commitment to marriage. Perhaps we should also be looking at how friendships can be expressed in covenants of commitment and intimacy. We’ll reflect more on this in a future essay. In the meantime, I will be using my new definition for spiritual friendship. I think mutuality and intimacy are integral to spiritual friendship and should be stated explicitly.

(1) The Four Loves [London, UK: William Collins, 1960], 78).
(2) Forbearance [Grand Rapids, MI: Eerdmans, 2017], 111.