Deep down Leon is a hopeless romantic who is pretty sentimental, especially when it comes to you, the love of his life.
Leon's become used to not treasuring things due to the fleeting happiness and tendancy for things to go bad, often he feels like everything he's loved has been taken from him, and all he's left with is the bad memories.
You're different though, Leon knows for a fact that nothing will happen to you, he's determined for your relationship to stay strong, and if anyone or anything tries to hurt you...Well it's not going to go well for them.
Leon has a box full of odd souvenirs and trinkets to remind himself of you, though he's pretty confident in your relationship, there is always that niggling feeling that one day he'll wake up and once again the thing he loves the most will be taken from him, he figures if he at least has these memories he can keep you with him forever.
It's just a shoebox filled with the most random shit. Odd reminders of your dates or days you spent together, cinema tickets and notes you've written him, silly doodles you've both drawn together or games of noughts and crosses. He also has some photos of you both in the box, and he keeps one in his wallet.
Leon isn't overly materialistic, but this box is special to him, you don't have any idea about it though as he keeps it hidden, maybe one day he'll show you it, but he's secretive about it for now.