Harry Potter and the Sorcerer's Stone
How we cite our quotes:
"I went looking for the troll because I – I thought I could deal with it on my own – you know, because I've read all about them."
Ron dropped his wand. Hermione Granger, telling a downright lie to a teacher? (10.142-143)
Although Ron had insulted her earlier, Hermione shows her loyalty to him and Harry – and to their house – by lying to McGonagall about trying to find the troll. She explains it later as a responsibility to stand up for them just as they stood up for her, by coming to warn her about and defend her from the troll. This show of loyalty is, in part, what makes them such good friends later.
"Very well," Snape cut in. "We'll have another little chat soon, when you've had time to think things over and decided where your loyalties lie." (13.114)
Here, Snape is taunting Quirrell, trying to provoke him into revealing whether he knows how to get past Fluffy or not. Snape does this by questioning Quirrell's loyalty. Harry overhears this and wrongly suspects Snape of evildoing, but we can totally see why Harry would think that.
Malfoy told Madam Pomfrey he wanted to borrow one of my books so he could come and have a good laugh at me. He kept threatening to tell her what really bit me – I've told her it was a dog, but I don't think she believes me – I shouldn't have hit him at the Quidditch match, that's why he's doing this. (14.105)
In a double dose of faithfulness, Ron is being loyal to Harry and Hagrid by not revealing to Madam Pomfrey what really bit him. But he's also paying for the loyalty he showed to his family and friends when he tried to beat up Malfoy for insulting them.