He's an old man with a bad heart. He's also an expert locksmith—like the Houdini of locksmiths. He's (secretly) the father of a super-famous author. And he's (secretly, in another way) the author of The History of Love. All in all, our man Leo seems like a pretty impressive guy, right? So why does he spend his time watching pigeons and intentionally dropping things in the grocery store?
Leo has been through a lot in his life. He's the only member of his family to survive the Holocaust, and he only made it by living in the woods—sleeping in trees and eating bugs and other unimaginably horrible things. Finally he was able to follow his beloved to America, but when he got there he found out she was now married to someone else. Oh, and they had a son together who doesn't know that he exists.
So, yeah, Leo's abandoned and alone. And he stays that way for fifty years. Except, he creates an imaginary friend to keep him company, an old friend from Poland named Bruno who died many years ago.
There are a couple of really important things about Leo. Let's deal with them one at a time:
Leo is, in many ways, the idealistic image of a writer. He explains, "It was the only thing I wanted to do with my life" (1.14). Although he's pretty good at speaking, it's in writing that Leo's able to express himself most fully. Whether he's creating fictional worlds or describing the things he finds around him, writing comes naturally to him as a child, and it colors his life like nothing else thereafter.
Everything he writes is for his beloved Alma, so even after she leaves for America he continues writing for her. But his experiences running from the Nazis rob him of the will to express things in words (we can only imagine), and he stops writing altogether. It's only after his heart attack years later that he begins to write again, as a way of reclaiming his will to live.
But the difference between Leo the young writer and Leo the old writer speaks to the novel's four big L's (other than "Leo," of course): loneliness, literature, language, and love. He began writing as a way to connect to his Alma, confident that he could express his understanding of the world in language (lit, lang, love!). But the experiences of his life rob him of both his companion and his belief in the possibility of communication (sad times). When he finally returns to writing, his motive has changed drastically:
I did it for myself alone, not for anyone else, and that was the difference. It didn't matter if I found the words, and more than that, I knew it would be impossible to find the right ones. And because I accepted that what I'd once believed was possible was in fact impossible, and because I knew I would never show a word of it to anyone, I wrote a sentence. (1.18)
For the elderly Leo, literature reinforces his loneliness, brings out his distrust of language, and seems to ignore love altogether.
This spirit culminates and collapses when he's seated on the park bench, positive that death has arrived. He starts speaking more and more freely, not exactly concerned with finding the right words since he's so convinced he's on death's door. But, when he snaps to and realizes who Alma is, he returns to his original precision with language and worries about finding the right words: "I couldn't speak. I was afraid I'd choose the wrong sentence" (16.151).
The History of Love takes place in 2005. Leo was born in 1920, which makes him… carry the one… an impressive 85. He's pretty old. Oh, and he's already had a heart attack. When they finally meet, Alma describes him as "the oldest man in the world" (16.86), so he must look even older than his years. Again, that's no surprise, considering the rough life he's led.
But Leo is obsessed with death. Not only does he think about it all the time—sometimes worrying about what will happen and other times happily fantasizing about being reunited with his Alma and their son Isaac—but he's certain death is going to come knocking, like, any second. It's right there in the first lines of the book: "When they write my obituary. Tomorrow. Or the next day" (1.1).
In a way, we could see this preoccupation with death as a positive development for Leo, since for many years, after living in hiding and moving to America, he considered himself invisible, almost non-existent. And you can't die if you don't exist, right? So his other weird eccentricity—going out into public and knocking things over to make sure people notice him—is sort of a backwards way of finally embracing his identity, his existence. He says he does it because he doesn't want to die on a day that no one saw him, but that explanation doesn't really have much weight behind it (well, except his having been traumatized by the death of a neighbor who wasn't found for days).
In any case, by the end of the book his calls for recognition are answered. He begins to receive confirmation of his existence—for example, the return of his long-lost book, the publication of his story in a magazine (with him as the main character, no less), the letter "from" Alma. Leo's grasping for identity has been one-sided for so long, so in this development he can see only one possible conclusion: death has finally arrived for him. But, nope—Alma brings him back around, and really almost brings him back to life.