Explanation

The pattern in the series appears to be a decrease of 1, then an increase of 3, repeating:

7 -> 6 (decrease by 1)

6 -> 5 (decrease by 1)

5 -> 9 (increase by 3)

9 -> 3 (decrease by 6, but pattern suggests decrease by 1)

3 -> 12 (increase by 3)

12 -> 1 (decrease by 11, but pattern suggests decrease by 1)

1 -> 15 (increase by 3)

15 -> -1 (decrease by 1)

Therefore, the next number in the series is -1.