Quote 1
"I – I made a grave mistake in marrying Tonks. I did it against my better judgment and I have regretted it very much ever since… Don't you understand what I've done to my wife and my unborn child? I should never have married her, I've made her an outcast!" (11.64-66)
Lupin is tortured by the idea that he might have already ruined his unborn child's life, simply through his werewolf nature. However, Harry reminds him that a choice that's been made cannot then be unmade – after all, Lupin married Tonks because he loves her, and there's nothing he can do to get rid of that bond.
Quote 2
"I am sorry too," said Lupin. "Sorry I will never know him – but he will know why I died and I hope he will understand. I was trying to make a world in which he could live a happier life." (34.57)
Lupin, here speaking of his son Teddy, is sure about his choice to fight, even though it means that he'll never get to know his only child. He speaks with conviction, knowing that he did all he could to help build a future.
Quote 3
"Expelliarmus is a useful spell, Harry, but the Death Eaters seem to think it is your signature move, and I urge you not to let it become so!"
Lupin was making Harry feel idiotic, and yet there was still a grain of defiance inside him.
"I won't blast people out of my way just because they're there," said Harry. "That's Voldemort's job." (5.55)
Expelliarmus (the Disarming Spell) is Harry's signature move – and for a reason. As he comments, he's not willing to use violence against people who don't merit it, which displays the strength of character and sense of morality that Harry's developed over the past few years.