Ethos is the basic belief system characteristic of a group or individual. Knowing this, some of the most basic examples of ethos are those involving slavery. Huck, for instance, grew up believing that black people were slaves. There was no other explanation. However, Jim and others, wanted to be free since they were people. Whites in the south didn't recognize them as anything more than property...like the livestock they owned. This book helps Huck to realize that Jim is a living, breathing, feeling, thinking individual, and he comes to believe that Jim--not just a black slave, but now his friend--deserves to be free.
Another example is the basic knowledge of right and wrong. Huck does not allow the King and Duke to swindle the Wilkses because he knows the members of the family are good and righteous people whereas the King and the Duke are con artists.