While I would like to say loyalty, it somehow doesn't seem it fit as nice as honesty. Then again, loyalty does come in many shapes and sizes, and it depends a lot on one's point of view as well. A lot of things depend on one's point of view really.
As for my own, I would like to say the element of belief. A strong component in any positive relationship, and a valuable life tool as well. Some folks have lived their entire lives by their beliefs, while others cast theirs aside when they become inconvenient.