Apart from Aristotle, are there any other definitions of a tragic hero I should refer to when writing about Tragedy?
There are others, but I think you might find Arthur Miller's "Tragedy and the Common Man" to be helpful if you're writing a comparison. The "older" view of tragedy holds that the person has to be of elevated nature (there's more to it than that, but you can look at the last link below for more information on the Poetics.
Miller lives in a democratic society that tends to an egalitarian view of people and does not believe that an elevated character is necessary for tragedy. Here's a brief quote that suggests what he thinks:
I believe that the common man is as apt a subject for tragedy in its highest sense as kings were. On the face of it this ought to be obvious in the light of modern psychiatry, which bases its analysis upon classific formulations, such as the Oedipus and Orestes complexes, for instance, which were enacted by royal beings, but which apply to everyone in similar emotional situations. (from Tragedy and the Common Man)
If you are familiar with any of Miller's work ("Death of a Salesman" or "All My Sons") you can see how this works out. Since every person, regardless of his/her status in life, seeks validation and meaning in life, everyone is a subject for tragedy when they are frustrated in their efforts.
Here's how Miller ends his essay:
It is time, I think, that we who are without kings took up this bright thread of our history and followed it to the only place it can possibly lead in our time--the heart and spirit of the average man.
Hope this helps with your essay!