To help you get started on this question, let's brainstorm some characters that Mark Twain might admire in The Adventures of Tom Sawyer. Remember, though, that each of these characters is complex and that some of their traits are admirable and others are not. Twain likely admired (and hoped his audience would admire) these characters only to a point, for just like real human beings, they are not perfect.
Tom Sawyer himself certainly has some admirable traits. Tom makes plenty of mischief, but he also has a good heart underneath it all. He feels guilty, for instance, about Muff Potter being wrongfully accused of murder, and he eventually speaks up to testify in his favor. Tom also cares quite a bit for Becky and protects her when they are in the cave, even bravely finding a way out for them both.
Tom's aunt Polly, although she struggles to keep Tom under control, is admirable because she really loves her nephew and wants to raise him right. She is a simple, kind soul who tries to strike a balance between the discipline Tom needs and the freedom he wants.
The Widow Douglas is also a kind soul who is a friend to local children, even giving them ice cream. She wants to adopt Huck Finn even though she knows she will have her hands full with him.
Huckleberry Finn himself has some admirable qualities for all his trouble. He acts to save the life of the Widow Douglas after he hears Injun Joe's plans. Huck is a nuisance, but like Tom, he has a good heart.
Finally, Tom's cousin Mary is a sweet young lady who is fond of Tom and acts with affection toward him.
Now, with these ideas in mind, you can expand upon these characters to complete the assignment. Go back to the novel and pull out specific quotations or examples that show Twain's admiration for these characters in their actual settings and events.