Your name is 'Nathan Simpson', maybe, as i just guessed you put the 'k' when signin up. The last name could be '심' from 'SIMpson'. That'd be fine, as most of Korean have one-letter surname. The first name could be '나단' from 'Nathan' itself. It doesn't sound very similar though. If you wanted to be called like 'Nat', no good idea. But '나잍' ,as Mr.Chopinxenakis said, or, '나일' to make it natural like a korean name. I think 심나일 sounds slightly better than 심나단. =) BTW. I don't think you have to rename yourself in korean right now, unless you will become nationalized. If you got korean co-workers or friends in person, they would give you a fit name as they 'd felt. that'd be better .. =)

how about 나잍 instead of 나이트? "나이트 성심" sounds like a night club meaning a disco club. You will not want to use it if you know the image of this word in Korea. But, "나잍" sounds cool and dosen't remind me of any direct meaning.

성심

How about just 기사 ? 기사 means a Knight.

성심 sounds a like a female name. how about "충심", or "충의"? It sounds heavy and manly. Both also means fidelity. I prefer "충의"