I would disagree with the below view. I have read the Mahabharata in original Sanskrit and Sri Krishna's teaching in the BG appears in several other places as well. In fact it would make a good argument to say that the entire Mahabharata from beginning to end in various places touches upon philosophical narratives relating to the Karma theory we find in the BG. As for the Krishna character, even if we remove the BG, his personality and dharmic orientation suffers no harm, indeed he figures as a truly gigantic intellect, always striving selflessly for dharmic goals. If anyone would like further light on this, I can post specific instances, although the MBH being a vast ocean-like work it is not at all an easy task to comprehensively bring out the various instances where Sri Krishna shines as mentioned above. But really from the time he recognizes the Pandavas as the 5 brothers who are seen together at the Swayamvara of Draupadi(even before Arjuna breaks the bow), Sri Krishna starts his role on behalf of the Pandavas as well as dharma. One of the Advaitin Gurus, Sri Shankaracharya, has always referred to Sri Krishna as Bhagavan Krishna and in his Gita-Bhashya, one of his most rounded introductions to a Bhashya, he goes into a lot of reasoning and detail on how Sri Krishna was God incarnate who appeared in human form for the protection of dharma, as well as of Brahmanatva. Excellent introduction by a revered scholar. appears