Origin
He served the Worthington family as their physician throughout the years and is one of the few people Warren trusted with his superhero identity as the Angel. When Warren was born Doctor Stuart knew he was born different and that he’d grow up abnormal.
X-Men: The Hidden Years
Years later while Warren was attending Xavier’s School for Gifted Youngsters he teamed up with Warren’s Uncle the Dazzler. He supplied Burtram with a drug that would make people more malleable while also slowly killing them. The pair’s plan was to have Burtram marry Kathryn dosing her tea with the drug to make her more open to the idea. She was already in a lonely state to begin with having her son away at school and her husband dead. Burtram eventually got too impatient when Warren and the other X-Men showed up and ended up overdosing Katheryn. After the X-Men stopped the Dazzler he spilled the beans about his plan and they confronted Stuart who admitted the reason for his deception was that he was prejudice against mutants and had regretted not killing Warren when he was born for he knew what he was going to become.