Mordechai is truly one of the good guys in the story. Without him, it seems that the whole of the Jewish nation would have been destroyed.
It is said that Mordecai was the uncle of Esther. Although there are several versions of exactly who he was and how they were related. Some say that they were actually married for example.
It seems that the major role that Mordechai really played was just having faith the whole time and being able to guide others on what they should be doing. For example he told Esther exactly what to do in terms of inviting the king and Haman to a party.
At another point in the story, he overheard two of the Kings men plotting to kill the king. It is unclear exactly how the that the king know but he seems to let the king the and the king gave him a massive reward for that. Without Mordecai, the story of Purim would've been very different.