I won't repeat was was said above, only add to it. "Buddy" seems to be used more by men, when referring to male friends, or to specify that the friendship is based upon a shared interest, in this case also used by females; for example, "He's my fishing buddy" or "She and I are yoga buddies." "Partner," in addition to referring to someone with whom you work, is frequently used among same-sex couples who are not married when referring to each other.
"Cause" is also a noun, meaning the reason for something: "Her careless driving was the cause of the accident." It is also a verb, meaning to make something happen: "Did you cause the accident by driving too fast?"