I was first exposed to MBTI through Death Note, the internet loves figuring out character using Myers Briggs test and the character of DN are PERFECT for that.
Light is considered ENTJ for example, they are said to be leader material - assertive, outspoken... Light makes me think about Fen'Harel, well, before he became a loony.
L, on the other hand, is an INTP, said to be individualistic, have no interest in leading people, value knowledge, etc.
Rem is INFP, who are considered the idealists - quiet, reflective with a strong value system.
L makes me think of Solas, easily, very cunning, tad eccentric.... Sweet-tooth -cough-. Solas also has that idealistic side to him, and he likes helping people... That's why he is INT/FP for me.
While Fen'Harel is a mask, there is a bit of him in original Solas as well, specifically the vanity (pride)...
Light in his arrogance thought he could rid the world of crime, he tried to change the world by killing criminals and even though his (initial) goals were noble, his methods were horrible. The notebook affected him and accentuated his narcissism and drove him to megalomania after he decided the only way is to create a new world and become its' god. I'd say Light is a perfect equivalent to a misled spirit of wisdom turning to a pride demon. Kind of disturbing when you think about it...
...
Solas, ugh, no, STAHP.