Abyss is a dragon who fused with Echoes DeLune in ancient times to save her from death, having been sealed away alongside her and brought to the modern day. While the nature of the life he once lived is unclear, he was once a king of his own civilization. However, he seems content with his new life alongside Echoes. He shares in Echoes's playful irritation at being compared to a Jojo's Bizarre Adventure character.
The primary reason Abyss has his own page is to discuss the difficulty of determining whether or not to count him as his own, separate member. On the one hand, he is a separate person with separate thoughts and feelings from Echoes. On the other hand, however, the nature of them being bound together means they couldn't exactly be far from each other, even if they wanted to. He seems to possess the ability to recede into Echoes, effectively "turning himself on and off" as needed. Thankfully, since this system is no stranger to ambiguously-counted members, Abyss being anomalous like this isn't a problem, and it can be left up to personal interpretation whether he counts as his own member or a part of Echoes.