Within Boyfriend's mind after Daddy Dearest falls to the corruption, A loud roar forcibly wakes Boyfriend up. Finding himself within the center of his mind, he meets the source of the roar - a black, demonic entity resembling him known as Soul Boyfriend - who immediately challenges him to a rap battle. At first, it would seem that Soul Boyfriend is fighting on the side of the corruption, not helped by his egotistical behavior: throughout the first song, he mocks and taunts BF, saying things like "too easy" and "not enough".

More facts here: https://fridaynightfunking.fandom.com/wiki/Corruption#???