His name is Theo, a military soilder, whom enlisted into the Army at the age of 18 and has had to take life threatening missions just to survive in the harsh world of military life, has survived many near death experiences, climbed the army ranks and becoming a major at the age of 29, was his biggest accomplishment, before he had to retire because of health issues, he was living the best of his life after his retirement, watching movies, comics and so much he hadn't been able to enjoy since becoming a soilder, but he was disappointed at not being able to ride his motorcycle, because of his health issues, alas, his time came when an assassin broke into his house and kill him, not being able to do anything, because of his health.
And that is when his rebirth into the marvel world started, he didn't gain his memories right off the bat so he grew up thinking that his world was normal, joined the police force at 18, became S.W.A.T officer at 21 climbed the ranks and became a comander at age 23, led his team to many succes for two years before he submitted a form of retirement, nobody knows why he did that, but after all the great deeds he had done, his form of retirement was accepted, and thats how he became the youngest swat officer to retire from duty at the age of 25.
he moves back to London where he was originally born in which he was called up by shield a year later, after retiring from swat why would he join shield?, well that's for you to find out lol, seeing no reason to refuse, he accepted recruitment, this is when Leo received his previous life memories.