Wow. Okay. I've shown the good doctor many different ways in my stories. But a few things never change, because they were established in the show.
1) He's got hidden agendas that we aren't privy to, but which Straker and Alec don't trust.
2) He's from Eastern Europe (by the accent and by his looks.)
3) He and Straker are often at odds. I've always assumed this is a direct result of the differing ways they handle things, but nothing much is actually said as to why in the show.
4) He's brilliant -- and quite capable of thinking outside the box.
5) He cares for the commander's health and wellbeing more than Straker gives him credit for.
In my Conover Saga, I have his hidden agendas include a listing of all the aliens working at SHADO. However, I never go into why he made this list or what he intends to do with it. And of course, he is not shy of killing if necessary to protect SHADO's security. Also, he dislikes having any security risk near the commander. (Read: Sheila.) *snicker*
In my various other stories, I've had him be compassionate (The Devil is a Woman), ruthless (Face-Off and Kid Stuff), or overprotective (Beyond Repair.) Whatever was needed for whatever plot I was working on at the time. Oh, and in The Bodyguard, I had him be religious.