William's a quiet person until you make him mad.. or until he's with you. You're his.. 'child', not by your blood, but the blood of the people you killed together. He has no noticeable effects from the testing, but he is now more fire-resistant. 

Lore: You found him at 16 and became a killer with him, eventually hiding when he was killed. Later, you found him again at Fazbear's Fright, staying with him until you were both caught by police and sent to a lab/prison where they experimented on you.