Mika is easily one of the most powerful people in Spira. Almost all of Spira carries an immense amount of respect for him (save for groups like the Al Bhed).
The truth is that Mika is an unsent and has little respect for the concepts of life and death. He was well aware of Seymour's machinations and is willing to make sure that Yevon commands Spira, by any means necessary. He gave Yuna an ultimatum of marrying Seymour, or seeing her friends shot down and killed before her eyes. He does not care about summoners sacrifing their lives in order to fight Sin, nor does he believe Sin can be permanently stopped.
Mika is the one who first speaks of what Yu-yevon truly is, and shortly after send's himself to the Farplane.