This is basically a cleverly disguised version of the "describe your three biggest weaknesses" essay. Begin by describing your key strengths as a leader, outlining a few specific skills and personal characteristics that you will bring to the table. You should then turn your attention to three areas where you feel that your experience or abilities may be lacking, and provide specific examples of why these traits are critical to the success of your venture (obviously using the "business partners" metaphor from the essay prompt).
For bonus points, end the essay with a paragraph that reiterates your current traits (i.e., what you can offer to your MBA class at Kellogg) and describe how getting an MBA will help you address the "weaknesses" you identified. If you can tie all of this back to specific features of Kellogg, you'll knock the reader's socks off.