Leo is worth elaborating on, here's why.
Leo deserves to be written more about, and it's sad that a lot of fics don't include him. He's like a lost character that no one really cares too much about, considering he's an asshole in two out of the three scenes he's in. I have looked at many, many different sources and played the game all the way through seven times, and after analyzing him as a character I have come to one conclusion. He can change. I will be listing some reasons down below.
1. Addiction Destroys a Person.
Addiction is something that a lot of people struggle with even today, and while being addicted to certain things is the person's own fault for trying it in the first place, it can be a hard hole to climb out of. Leo was born out of a fling Carl had, he was raised by his mother and he just recently connected with his father when he was 16. Not having a father can bring on a lot of emotional trouble, ranging from trust issues, to anger issues, to resentment, to blaming issues, to self-critical thinking, and all of the above. So, when people feel these things, they really only have two things to turn to. Help and Temporary Treatment. Temporary Treatment can be self-harm, abuse of other people, destruction of property and items, and substance abuse including alcohol, cigarettes, and drugs. Leo, without knowing what else to do, turned to drugs, and of course, he got addicted. He got addicted because it was the only thing that was there for him. He got addicted because it was the only thing he had. And when Carl magically came back into his life, he, of course, felt a lot of resentment towards him.
2. He Wants to Honor the Manfred Name.
What I mean by that is that he wants to be able to hold the Manfred name and be able to live up to everything his father is. He wants to be able to be proud to be Leo Manfred, and he wants Carl to be proud too. When people suffer emotional issues they tend to become more perceptive, maybe more anxious about things around them. This means that A. he knew that Carl thought of Markus as his son from the beginning, or B. he was anxious about it becoming true and him being replaced, which if Carl survives it does become true. He grew up in a society with Androids, in a society where Androids are marketed as a replacement for everything. Workers, Partners, Children. He's scared and he lashes out at Markus because he recognizes that Markus is an infinitely better son than he is. He recognizes that he's a lowlife who's hooked on drugs and just takes from a father he barely knows or appreciates. He wants to be Markus, as perfect and proud as he is able to be, and yet he isn't. He can't. A lot of his anger and violence is from pride.
3. He Wants to Change.
Now depending on whether or not Carl is alive or dead by the Final Protest, he will be able to show signs of wanting to change his ways. Before I analyze his video message, I want to analyze him coming to his father's grave. He exchanges a glance with Markus, and at first, I saw it as recognition, but I realized it was resentment. He resents Markus, he literally and figuratively pushed all of the problems of his actions onto Markus by blaming him. In his mind, if Markus was never there, none of this would ever happen. This scene is different from his video message because, in this, he's a lost cause. If Carl lives he goes to the hospital and has a chance to repent on his actions, and he realizes that he has to make a vital change to himself to be able to be proud to be his son. He understands this and he has someone to change for, Carl. If Carl dies, however, he has no one. His mother is no one special, he isn't fighting to be proud to be her son like he is for Carl. If Carl isn't there, he never repents, he never decides to change, he gets a butt load of inheritance money and assumingly uses it to fuel his addiction. He isn't going to Carl's grave to make amends, he's going to Carl's grave to be angry. Angry that he died, angry that he gave all of this money for him to destroy himself with, angry that he wasn't there for him as a child, angry that he indirectly led him down this path. Most of all, it's a pity party. He's angry at himself. But, if Carl lives, he finds it in him to want to change. And that is the most hopeful thing about him, is that he as a character can change. If you play the entire game correctly, getting the best ending, you will see his video message. He will change. Because it's a long journey, and he knows this, but he's no fool. He knows that he's the one with the problems and that he's the one to fix them. And that's why more elaboration on him as a character is so crucial, to see him do this and change into a better, possibly entirely different person.
Leo, despite what you see of him in the game, is definitely worthy to be written more about. He is possibly the only character in D:BH that has this level room to grow. Sure, there's Gavin. Sure, there's Elijah. But Leo has proven throughout my research to be the first and only character out of those three that show a willingness to change and become a better person.