TALE OF A CANTERBURY TALE

Alright so we got the Monk right? He tells us he's going to "bewail" (or express with great regret) about people who essentially fell from grace, previously being at a "high degree" or very important. He begins with the fallen angel Lucifer and how he was once an angel and got the banhammered  from heaven by Bog himself, and now rules hell as Satan. He lists a couple of other Biblical figures such as Adam and Samson (who kicked the asses of three-hundred foxes in one occasion and a thousand men in another). Adam fell from grace through their own "misgovernance". Samson spirals down a path of self-destruction and starts drinking and falls in love with a harlot (Delilah) who cut his hair, which was his source of power. He then collapses a temple on himself and his captors. The Monk goes on to talk about Hercules and Nebuchadnezzar (king of the Neo-Babylonian Empire) and how they met their doom at the hands of a woman and their own arrogance, respectively. Nebuchadnezzar's son Belshazzar got drunk and makes some stupid decisions which lead to his death. He goes on to name a bunch of other historical figures such as Julius Caesar and Alexander the Great and how their own cockiness, shortsightedness, and stupidity led to their downfall and/or death.

1. There really isn't a main character in the Monk's tail. His focus changes from a plethora of Biblical and historical figures. By referring to a good amount of Biblical characters through his story, you can obviously tell the Monk is extremely religious (if the fact that he was a monk didn't already tell you that). He's well-versed in both history and the Bible; this is evident when he says "...tragedies I'll tell, Of which I have a hundred in my cell...". You can tell he is a bit of a joykill and 
2. I believe Chaucer wrote this to serve as a warning or a lesson to his readers. The lesson here is not to be overly vain or it will lead to your death, as seen with Nebuchadnezzar.

This king of kings was arrogant and vain; He thought that God who sits in majesty Would never take from him his great domain. But he lost that dominion suddenly, And after like a beast he came to be: He ate hay like an ox and lay about Right in the rain, wild beasts his company, Until a certain time had run its route They all strut their stuff and got carried away, believing that they were invincible and unstoppable. (lines 2167-2174)

or with Samson...

Before his hair had thus been clipped away, Men simply had no bond, this man to bind; Now he's imprisoned in a cave where they Have bound him to the handmill, there to grind. O noble Samson, strongest of mankind, Once judge with glory, wealth, and blessedness! Well you may weep with eyes that now are blind, To fall from where you were to wretchedness. (Lines 2071-2078)

All of the characters in the Monk's series of tragedies put their trust in blind prosperity and don't acknowledge the fact that fate is a mercurial thing. If anything, the Monk is the pessimist of the Canterbury Tales. He puts a damper on things by telling a story which has kind of a somber undertone.