As per Enotes rule, we only answer one question at a time, but I will tie in this one since the two questions have some common ground.
There are several instances in To Kill a Mockingbird that illustrate the many ways in which Bob Ewell is racist and aggressive. He is racist because he is at the absolute bottom of the social ladder in Maycomb, having been born into a family that, for generations, has been associated with chaos, trash, crime, and a lot of horrid and unhygienic behaviors.
This being the case, the only two types of people who would have ranked even lower than Bob in the social "food chain", due to the overall racial ignorance of the people, would have been African Americans and women. Therefore, Bob picks precisely on these people (and later on children), to feel superior. He may have been born and raised as a racist, but his racism also serves as a tool that he uses to bully those who are socially "beneath".
A clear example of racism happens on chapter 17, when Ewell formally accuses Tom Robinson of raping his daughter Mayella:
"I seen that black nigger yonder ruttin' on my Mayella!"
There is a lot of power packed in those words. First, the use of the "n" word alone would have been bad enough, as the word often associates the receiver with crime, dirt, and everything bad. However, he also passed by the skin color of the man, making it a double attempt at insult.
The use of the words "that black n....yonder", instead of calling Tom Robinson by his name, makes the accused to look even less human, and less worthy of being there. It is as if Bob was not worthy of mentioning Tom's name because he (Tom) is beneath him.
Additionally, Bob uses the word "rutting" to add more humiliation to the situation and to the accused. A "rut" is the procedure by which two animals are made to mate. Therefore, he is also calling Robinson an animal that wanted to force himself into an innocent Mayella--which we know is not the case as it happened at all.
Bob's aggressiveness is both passive and active. It is passive in the way that he likes to intimidate with his stand-offish presence alone. It is active when he:
- spits on Atticus
- hits and abuses his own daughter Mayella
- threatens the judge
- enters the judge's house to either kill him or do something else
- goes after Helen Robinson and bullies her
- goes after Atticus's children to kill them in cold blood
So, yes, Bob is racist, abusive, aggressive, and a huge bully.
Jem, on the other hand is courageous in many ways, but not as openly as his sister, Scout. One thing to consider is that Jen and Scout had to defend themselves against the very Bob Ewell described before. Jem was attacked first. As a good brother, he yelled and hollered at his sister to get away. She was not able to get away, but then Boo Radley took care of the situation.
Whether it was warning or preventing his sister from the dangers of Maycomb, Jem always had the courage to show that he would be there to defend his people. This is even more evident on chapter 15 when he stands up for his father in the presence of the lynch mob that went to the jail to kill Tom Robinson. The fact that Jem refuses to leave shows that he was willing to do whatever he had to do to protect his father. We learn later that Atticus was actually quite moved by this, as he reached out to Jem on the walk back to the house, and kindly touches his son in a sign of affection and gratitude.