Loyal friends make for some of the most memorable characters in literature. (Certainly they're amongst my favourites). Some of my favourite character traits are shown - determination, loyalty, courage.
So what makes your characters want to "do their bit"? Were they brought up to be loyal or have they known betrayal and so are determined not to betray anyone themselves? Are the friends they are loyal to grateful? In The Lord of the Rings Frodo generally is grateful for Sam though I found it interesting that their whole relationship is put under enormous strain, and nearly breaks, thanks to the evil influence of that ring. But loyalty proved to be the strongest trait (which is one reason why I love this book).