The pattern of the tragic hero was first defined by Aristotle. Aristotle's work The Poetics discusses the art of Greek tragedy, and defines the rules for a tragic protagonist. If we examine these rules from Aristotle alongside the Medea of Euripides, we may see how Euripides observes or breaks the classic pattern. I suggest that Euripides observes more rules than he violates, to better emphasize those aspects in which he differs from the Aristotelian norm.
The first part of the pattern of a tragic hero is discussed by Aristotle in Poetics Chapter V. Here, Aristotle defines what will come to be known as "the unities":
They differ, again, in their length: for Tragedy endeavors, as far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit, whereas the Epic action has no limits of time. (Poetics V)
This means that a Greek tragedy will always have (or should have) the same basic structure: the play has a "unity" of time, because it all takes place within Aristotle's "single revolution of the sun." The tragedy additionally features a unity of place (i.e., there is only one set) and a unity of action (i.e., there is only one plot). The Medea of Euripides observes all these "unities" -- the whole tragedy takes place in a single day, in one location outside Medea's house in Corinth, and it tells the story of Medea's revenge on Jason for deserting her. Even though Medea is not the first character to appear or speak, the Nurse's opening speech presents Medea to us by way of introduction:
Nurse:…While Medea, his hapless wife, thus scorned, appeals to the oaths he swore, recalls the strong pledge his right hand gave, and bids heaven be witness what requital she is finding from Jason. And here she lies fasting, yielding her body to her grief, wasting away in tears ever since she learnt that she was wronged by her husband, never lifting her eye nor raising her face from off the ground;
This immediate initial introduction means that Medea is the Aristotelian tragic protagonist.
The Aristotelian pattern for tragic hero continues with his outlining of the pattern of the plot. For Aristotle the tragic hero must have two elements in the plot: the recognition (or anagnoresis) and the reversal of fortune (or peripeteia). He defines the first one in Poetics Chapter IX:
Recognition, as the name indicates, is a change from ignorance to knowledge, producing love or hate between the persons destined by the poet for good or bad fortune…things of the most trivial kind may in a sense be objects of recognition. Again, we may recognize or discover whether a person has done a thing or not. (Poetics IX)
For Medea, this recognition occurs late in the play, as she recognizes that Jason has deserted her permanently. This resolves Medea to kill her children in revenge on the ex-husband, as she describes before exiting the stage:
O my babes, my babes, let your mother kiss your hands. Ah! hands I love so well, O lips most dear to me! O. noble form and features of my children, I wish ye joy, but in that other land, for here your father robs you of your home. O the sweet embrace, the soft young cheek, the fragrant breath! my children! Go, leave me; I cannot bear to longer look upon ye; my sorrow wins the day. At last I understand the awful deed I am to do; but passion, that cause of direst woes to mortal man, hath triumphed o'er my sober thoughts.
The fact that Medea then commits the murder is the reversal of fortune. These two elements show that Euripides observes the traditional pattern for tragic protagonist in the plot of Medea.
Aristotle's Poetics Chapter XV outlines the specifications for character in the pattern of the Greek tragic hero. These entail several different rules, which must be considered individually. Aristotle outlines the rules thus:
In respect of Character there are four things to be aimed at. First, and most important, it must be good. Now any speech or action that manifests moral purpose of any kind will be expressive of character: the character will be good if the purpose is good. This rule is relative…