Explain how Ichabod Crane is positive and comical on one hand and negative on the other.

In Irving's "The Legend of Sleepy Hollow," Ichabod Crane can be seen as a comical character first because of his appearance: 

He was tall, but exceedingly lank, with narrow shoulders, long arms and legs, hands that dangled a mile out of his sleeves, feet that might have served for shovels, and his whole frame most loosely hung together. His head was small, and flat at top, with huge ears, large green glassy eyes, and a long snipe nose, so that it looked like a weather-cock, perched upon his spindle neck, to tell which way the wind blew.

The narrator goes on to compare Ichabod Crane's appearance to that of a scarecrow, making his pursuit of the beautiful and wealthy Katrina Van Tassel seem ludicrous, especially when contrasted with his rival, Brom Bones, a young man "of Herculean frame" and athleticism.
It is also comical that a man as educated and intellectually gifted as Ichabod is so easily duped by Brom Bones into believing that the ghostly decapitated body of a Hessian soldier from the Revolutionary War stalks about at night and could harm him.  
What is negative about Ichabod Crane is his falseness, greed, and avarice.  He comes to Sleepy Hollow as a teacher, traditionally a "helping" profession. However, once he sees the bounty of Sleepy Hollow and the wealth of Katrina's family, he pursues her not for love, but for what he will gain from marriage into a wealthy family. He greedily helps himself to all the delicious and plentiful food his hosts offer, dreaming of the day he can abandon the schoolroom and be the master of a bountiful farm.