Roppi is potrayed as part of Izaya's inner psyche. In contrast to Psyche, he is the dark side of Izaya, suicidal and never really happy. He is cold, calculating, and quite often sees no point in living. Unlike Izaya, he actually HATES humans with all his heart. Emotions and everything about them he detests and he wishes they didn't exist.
Lots of fanfictions about Roppi portray him as mainly a suicidal character with cuts on his wrists and a strong hatred for the human race. Others potray him as quite murderous or just lonely. It is also shown that he has an abusive past, leading to his hate for humans.
Roppi is almost identical to Izaya except for the fact that the fur on the jacket is red.
Roppi's most common partner is Tsukishima. While Roppi does hate all humans, Tsukishima is an exception because of his pure heart, he's kind and sweet and is the only one that has cared for Roppi enough to stick around. He is the only person Roppi tolerates and their sexual life tends to be a lot softer than one would expect.
Roppi is also paired constantly with Izaya, although not as commonly as Tsukishima and Roppi. Izaya is shown to be hated by Roppi, as Roppi finds him irritating and annoying. It is also hinted in some fanfictions that Izaya is responsible for Roppi's abusive past, leading to Roppi's dislike for him. However, their opposite personalities and interests cause them to get attracted to one another.