Lei was born on Vulcan, but raised on Earth. His farther was Vulcan and mother human, his parents broke up soon after he was born, his mother took him to Earth and raised him there.
Lei's Mother loved each of her children and Lei became very dependant on that love, when she was killed at Wolf 359 he held the Borg and more importantly his farther responsible.
He passed school with a lot of high grades and honours at most school, his temperament caused him to get in allot of trouble at school. Later, he joined Starfleet; again his temper caused him problems. He got high grade's impressing allot of people but because of violent actions he was almost expelled twice. 
Lt. Lei Thomas has twice been reprimanded for abusive behaviour towards senior staff, he gives seems give no regard in flouting rank or authority when he believes he is in the right.
Apart from his stubborn attitude Lei is a very fine officer, a widely skilled engineer and a very good leader. When the Romulans stole the USS Triton, Lei took it back on his own since he was the only on still on board at the time, and for his brave act he was given the Christopher Pike medal of Valour.
Lei recently took the executive officer posting at Outpost Cousteau, he hopes this step will bring him closer to a command of his own ship.