When it comes to describing someone as a fast friend, there are plenty of synonyms to choose from. Some alternatives might include close friend, trusted companion, loyal ally, or confidant. Other options might include soulmate, kindred spirit, bosom buddy, or bestie. Regardless of the term you choose, these synonyms all convey the same idea - that someone is not just a casual acquaintance, but someone you share a deep and meaningful connection with. Whether you're discussing childhood friends, work colleagues, or even pets, using a synonym for fast friend can help to emphasize just how important someone is in your life.