Having dealt with quite a few (including starting a company with a well expressed one and being engaged to one)... the major qualities I've noticed;
1) Supportive of those that are in their "inner circle"... and they use anyone who isn't. And in general, they know this and take pride in it (or just accept that as normal).
2) Emotionally... naive. For a lack of better words, they have a real leap-frog approach to things. They don't like being uncertain. And emotions are pure uncertainty for them. My fiance is not the kind of person you go to for emotional support... my friend - hell no.
3) To make up for the emotionally part, they are relentlessly active in helping... in practical terms. When something happens to you, they will salt the earth of the opposing faction.
4) Unfortunately, this can happen to their "inner circle" when they decide you aren't worth their time. They tend to just cut people out, walk off... a very direct no-nonsense approach. At times this is good and needed... others, not so good. Letting them down is a good way for them to not consider you much of a friend. And letting them down can be very very subjective.
Having said all that, they make great mates. Generally smart, dedicated and resourceful... and they take any relationship they are in, friend and more, very seriously.