A summoner is someone the medieval church hires to call people before the ecclesiastical court for their spiritual crimes, like adultery or heresy, the punishment for which can be excommunication (expulsion from the church). Like many of the pilgrims, the Summoner's physical appearance is kind of cringe-inducing: his face is covered in sores that no ointment can heal and his narrow eyes are covered by fierce, bushy eyebrows. So hideous is his face, in fact, that children are afraid of it. On top of that, the Summoner likes to eat smelly vegetables like onions, garlic, and leeks, so his breath is probably pretty bad.
With the Summoner, we do not have a case of an ugly exterior cloaking a beautiful soul: instead, this guy is lecherous, dishonest, and generally unethical. He also drinks to excess, and when he's drunk he's very unpleasant to be around, yelling like he's "wood," or crazy, and trying to demonstrate his learning by spouting out the few words of Latin he knows.
Like the Monk, the Summoner is disdainful of the church's teachings, brushing aside people's fear of excommunication – "the erchdekenes curs" – with the knowledge that you can always pay a bribe to get out of it. And who do you think benefits the most from those bribes? Yeah, probably the Summoner himself. Beyond taking bribes, we also suspect the Summoner of seducing young girls; not only does he have them all in his confidence, but "ful prively a finch eek coude he pul" (General Prologue 654), an expression that can mean to trick, but also to seduce a young girl.
With the Summoner's portrait we have a critique not only of his individual character, but also of the situation that has created him. Although the Summoner's conviction that one can avoid excommunication by paying a bribe is morally reprehensible, it may also have been true. Historians also think that summoners were not paid enough money by the church to really make a living; thus, they may have had to depend upon bribery to get by.