One of the most important side characters are going to be a group of boys consisting of Leonhard, Jet, Walter and Richard. I have a lot to say about these boys though so I'm going to divide these introduction posts into four so each boy gets their own highlight. I love to ramble, you see.
Leon was also created while still considering Sinfonia a joke game, so I originally heavily based him on characters like Pascale from Kazeki and Oskar from The Heart of Thomas: older, taller and physically and mentally (more or less) more mature than the rest. However, he was originally going to be a supportive big brother type of character, but the more I worked on him the more I realized Leon is very irresponsible and definitely still very immature, even if he tries to fancy himself as more mature and smarter than the rest. Leon is the force that keeps people together and he does an excellent job at that, but he isn't exactly someone others turn on to when they need emotional consolation: the right man for that job is actually Jet.
Leon is considered popular in the internal hierarchy of the school, and as such he can say and do more things than many others would. This is partially because of his age (he would actually be in high school now if it wasn't for his long hospital visit) but also because Leon is generally very charismatic, fun and always ready to do stupid things under even the slightest of peer pressure. He is considered cool by the other boys and he takes a lot of risks, so it's easy for many other boys to admire him and wish to be like him. While Leon is still kind of a cool big brother of everyone in the school, he is reckless and constantly plays with his own life, even though he is protective of others.
He has strong moral beliefs and he can be merciless when it comes to forgiving and accepting people who have done him or someone else wrong. As such, he dislikes Claude greatly and along with Walter is the first one to speak ill of him out of distrust. Leon knows of the things Claude does because he usually gets information easily from others. Also, Claude is only slightly above him in rank which kind of frustrates him.
Although he is undoubtedly the leader type, Leon doesn't always pick up small signs and things from people and their behavior. This is something Jet, in turn, is very good of, so the two boys usually rely on each other to point out things the other doesn't see. They're really close friends despite their drastic differences in nature.
Leon used to know the boy who disappeared as they were room mates when Leon was a second year, and although he doesn't really voice his concern, it's clear he is bothered by the disappearance.
... except that I consider the sketches below even more accurate to his features
However, this is why Leon still changed a lot from my original plans: he used to be much more reliable and big brotherly, but then I thought he'd be interesting as a more flawed and untrustworthy person. I mean, he is a teenager and when I was 16 I sure as hell wasn't all that reliable even if I thought myself as one, so I decided to tone Leon down a lot too. I even though of making him 15 after all and not include the whole "broke his leg while doing stupid jackass shit and landed in hospital and was held back for a year" but in the end it felt like such a crucial part of him I just couldn't let it out. And really, considering just what kind of hopeless risk taker Leon can be, I thought it'd just make sense he would get badly hurt at least once - and learn nothing from it, as he is still doing his stupid stunts just like always.
It's true though that Leon can be reliable and mature when he wants to, and his own early puberty, his family situation and the long time spent in the hospital have shaped him, but his nature as a foolish risk taker just kind of tends to take him over fast. He is still young and his nature is so strong and overwhelming that his impulses are also strong, and he needs more time to work on that to get himself in proper control.
I like to think he will be a good adult though, especially after the events of Sinfonia. Leon is very dear to me as a character for variety of spoiler-y reasons (and also the fact his name comes from a lion, gaooo) so I really want to see him growing up both in Sinfonia and works I hopefully continue making after it. There is a lot he still lacks and he is not necessarily the smartest of the bunch, but he really does have a good heart and he cares for others really strongly.
I really enjoy writing Leon and Rick interactions, by the way. Because of their huge difference in age, size and maturity, their interactions are kind of unbalanced but still very... wholesome. I really love writing sibling-like relationships, and with two not-so-bright stubborn idiots it's even more fun.