The Dark One child if you choose to save Pavel despite his betrayal: "He'll live...so that's what forgiveness is...thank you, I will remember this."
While going into Polis, the Dark One child is disguised as a regular boy and will wave at one of the soldiers. The soldier responses with a small salute.
While wandering the surface with the Dark One child, he'll at one point decides he wants to wear clothes like you. A minute or so later, he appears to show you his new outfit. Its the cutest thing ever.
In the bad ending when D6 detonates and Artyom is having a flashback of all the important people in his life, he finally recalls his mother's face. It's bound to pull heart strings on anyone who's read the Metro 2033 novel as Artyom's inability to recall her face is one of his biggest sources of grief.