That’s smart of you to try to think of a quality that isn’t the usual: leader, determined, etc. and get one even more specific and unique to who you are. Brava! I like the idea of spontaneous (impulsive may be the negative side, sometimes a problems in itself…but if you can also show the upside!). Start with an example of a “time” you were impulsive, then give background explaining your tendency to be this way, then explain how you handled it, talk about the good and bad of your quality (with other smaller life examples) and what you learned about yourself. Done. Great essay!
Some applicants may ramble on about themselves in a manner that may appear self-indulgent and not very appealing to the committee. Remember, this is an application essay, not an autobiography. Conversely, some applicants tend to say too little, perhaps hesitating to promote themselves too explicitly or not knowing what about themselves would be interesting to people whom they don't know. In such cases, perhaps focusing more on what you want to do than on what you have already done (let your record speak for itself) may help in getting beyond self-inhibition.