Prior to the alien invasion, Dr. Tygan was a biochemist in Earth's lucrative pharmaceutical industry. Once the aliens conquered Earth and toppled all governments, Tygan didn't know what he should do. As the ADVENT administration was established and new cities were erected around the globe, Tygan's curiosity provided him a new purpose: he would seek access to ADVENT. He was then quickly identified for his skills and employed within ADVENT's ranks.
Tygan led one of the earliest Gene Therapy clinics for ADVENT and was impressed with the medical advancements the administration provided. Though Tygan was initially satisfied in his work, as time went on he found himself quite troubled with how ADVENT had applied his efforts. He saw increased control exerted over the human populace and, upon hearing of a growing resistance movement, decided to flee the city and seek out XCOM.
As can be seen in some cutscenes, Tygan has a large scar on the back of his head, indicating that perhaps ADVENT implants all of its associates with some kind of chip, which he forcibly removed when he fled. Dialogue from Shen hints that the chip may not have been removed completely, as Tygan scratches the scar more frequently than normal.
Dr. Tygan is self-assured and confident in his own knowledge and expertise. He can be a bit patronizing at times to those who aren't able to keep up; however, he has a great deal of respect towards the commander.