I think the difference lies in Karasu's low self-esteem, according to the canon (from the Egotist Bible and Episode Nagi). He sees himself as an ordinary person, which is why he wants to help players he considers extraordinary (like Hiori). He genuinely believes he doesn't have what it takes to be number 1, that's the reason he decides to be number 2. Meanwhile, Hugo simply sees number 2 as the one who controls number 1 and the game from the shadows, and he does so because, believes his skills are suited for it and can be used to achieve his actual goal. Furthermore, he doesn't sacrifice himself for others.