1 ) "可我却是个性格开朗." is a wrong sentence because of a wrong logic. 2 ) "可我却是个性格开朗的人" has a clear and correct logic and other logics are wrong. 3 ) "可我却是性格开朗的" has a clear and correct logic and other logics are wrong. 4 ) "我" should be a person or the predicative should be something about this person. "我"is "性格开朗" doesn't collocate with each other. Just like we cannot say "a person is greatness", but we can say "a person is great".

1) "是个"means "is a". It's not a word. "可我却是个性格开朗." should be "可我却是个性格开朗的人" = "but I am an optimistic man" or "可我却是性格开朗的" ="but I am optimistic" 2) And yes, "却"is a frequently used word, and most of the time a coordinate conjunction which means"but" or "while" or"however". 3) To understand correctly a Chinese sentence, you have to analyze the structure, get clear where are the subjects, the objects,predicate verb,predicatives,link verbs,qualifiers, determiners,adverbials and some other elements. Pay attention to the logic of sentences. The meanings are usually expressed by the logics.