In the Epilogue (Biohazard 4 Kaitai Shinsho novel)
Leon refuses to expect the worst from Ada because he has firm faith in her character. It shows that even though they follow opposite paths, his faith and trust in her remain unshakable, even without knowing about the countless times Ada risked her life for him:
"The sample of the modified Plaga was stolen by Ada. The seeds that could once again give rise to beings like Sadler still remain. But Leon isn't pessimistic. He believed the likelihood that what had fallen into her hands would lead to the worst-case scenario was low. Deep down, he was convinced that Ada was acting with a different purpose than Wesker.
Leon and Ada are now far apart. Though their hearts may be connected, their paths in life run parallel, stretching far into the distance.
There may be a future where their paths cross again someday. As long as they're alive, as long as the world continues.
—I'll make sure it continues. I've regained my reason to fight. For that, I'll walk through hell as many times as it takes."