Person
Ubayd Allah ibn Abbas
عبيد الله بن العباس
Lifespan unknown Quraysh
Ubayd Allah ibn Abbas was the commander of Hasan's vanguard who deserted to Mu'awiya, facilitating Hasan's abdication.
Significance
Commander who defected from Hasan to Mu'awiya
Sources: Wikipedia and classical Islamic biographical literature compiled by automated researchers. Every page is being continuously refined — if something looks off, please check back in a few days.