"The filthiest human" with a "dark gray" neck is Burris Ewell. Scout meets Burris briefly on the first day of school, and she describes him as having rusty hands with completely black fingernails. 

Their teacher, Miss Caroline, wants to dismiss Burris for the afternoon so that he may go home and wash his hair with lye soap and treat his scalp with kerosene. Burris doesn't understand why this is necessary, and when he asks Miss Caroline why he needs to bathe, she responds that it's to get ride of the "cooties" so that the other children don't catch them. 

Burris laughs "rudely" at this requests and asserts that Miss Caroline can't send him home because he already "done [his] time for this year." The Ewells only show up to the first day of class each year and then never come back because they have no mother to take care of them properly, and the "truant lady" struggles to enforce the rules.

As is tradition, Burris leaves that day with no plans of coming back, shouting that, "no snot-nosed slut of a schoolteacher ever born c'n make me do nothin'!" 

Burris Ewell is known as

"the filthiest human [Scout] had ever seen.  His newck was dark gray, the backs of his hands were rusty, and his fingernails were black deep into the quick.  He peered at Miss Caroline from the fist-sized clean space on his face."

Burris is the boy who only attends the first day of class and then never comes to school again.  He is the first of the Ewells that we meet.  The rest of the story is based around Mayella's (his sister) accusations on Tom Robinson and her father's (his dad) behavior and relationship with those in Maycomb.

This description happens in chapter 3 at school.