Born into royalty, where he learned the arts of sorcery and swordsmanship, Leonhard received horrid burns across his entire body through unmentioned circumstances. His face in particular was scalded terribly, and he wore a silver mask to conceal this.
Some unknown while later, he traveled to the Cathedral of the Deep, seeking out a way to heal his scarred body by rebirth. When he finally met Rosaria, he abstained from healing his burns for unknown reasons, instead taking vows to serve Rosaria and inheriting a crescent sword imbued with the power of the moon.
"Mm, Unkindled, are we... And fast on the trail of the Lords absconded? Then these red eyes are for you."
"Use them to pillage embers, and briefly heighten your strength, for your duty.
What else are Unkindled ashes good for?"
After a Pale Tongue has been obtained
"Well, hasn't it been some time. I'm Leonhard, the Ringfinger. Remember me? I slipped you those red eyes some time ago. You're making quite an effort of it. So I thought you might like to know. If you yearn for a proper red eye orb, one that is uncracked... Then kill the Darkwraith, survivor of the land swallowed by darkness. He has been a prisoner, for many ages. In the deepest cell in all Lothric..."
"I can see it in your eyes. If you didn't invade, didn't pillage, whatever would you do?"
After the Red Eye Orb has been obtained
"Ah-hah, you've found a proper red eye... Brilliant. I knew you were no ordinary man/woman."
"Now invade and pillage all you like. And if you grow weary of your duty, you, too, may become a Finger. Come on, give yourself to Rosaria, of the Cathedral of the Deep."
After joining Rosiaria's Fingers in Rosiaria's Bed Chamber
"Ahh, so you've chosen to serve Rosaria after all. She will be pleased with me for finding her another Finger... But be warned, my friend. Rosaria's Fingers need only fetch tongues for their mistress. Otherwise, we are free, unchained. Like Yellowfinger, you can choose to believe that all Fingers share camaraderie. But do not force your romance on the rest of us."
"What's wrong with you?"
"Can you even hear me? If you can, cease this at once. Before it's too late."
"I see, I see, as clear as day. You are a vile, starved beast. Much too dangerous for her!"
Killing the player
"You beast. You've trifled with the wrong knight."
"Curses, I can't... die here... My dear Goddess..."
When invading him in Anor Londo
"Well, well, never expected to see you here. Felt sorry for the poor thing, in all her festering glory? And now you want to ravage her soul, as well. I sowed the seeds. I'll prune the mess. I, Leonhard, swear so upon my vows to the goddess."
"No one will despoil her soul. Certainly no beast wrapped in human skin!"
"What do you want with her soul? Isn't flesh enough for a sick beast?!"
Killing the player's phantom
"Enough with you, my friend. A knight of the goddess does not die."
When invading him in Anor Londo having been previously killed
"Well, well, back for more, are you? Vicious one, aren't you? I knew it all along. And now you want to ravage her soul, as well."
"You are no longer of use. You'll run free no more. I, Leonhard, swear so upon my vows to the goddess."