The woman in the final scene is Elliot's mom and the kid is Elliot. The third personality is confirmed not to be the child version of Elliot based on the answers his mother gave Child Elliot and that office seems to be a visual representation of Elliot's mind and the desk/chair is occupied by the personality that is running the show in Elliot's head at the time.
I'm putting money on it now, Elliot didn't properly mourn Angela. So I think the way he coped with her death was that he created a personality that is the direct representation of Angela. Elliot's mother was kinda heated when she said "the other one", remember she had kept that cassette of the Happy Mother's Day recording that Elliot, Darlene, and Angela created for Angela's mother so she has reason to resent Angela. What other person has just as much reason to take down Whiterose? Fucking Angela.