Gu Family Book perhaps? I don't think the SML ever confesses to the FL, but he definitely tries to keep her apart from the ML. The ML is more than just eccentric though. He's half gumiho.
Because This is My First Life is also close, but there isn't really a SML pursing the FL. There are two characters that pursue the FL at different points apart from the ML, but neither of them is quite prominent enough to be called the SML.
For Cheese in the Trap, the ML is very good at hiding his "eccentric" nature (it would probably be more accurate to say the darker aspects of his personality), so he's not looked down upon by everyone until that side of him is revealed. But the SML knows and strongly dislikes him since they had a falling out in their high school days.
