McGuinty is a minor character who appears early on in the book. He is mainly important because of the contrast he presents to Jacob. McGuinty seems to like the old people's home and wants to fit in there, in contrast to Jacob, who hates it and wants out. McGuinty goes so far as to tell a lie about working for the circus, while Jacob, who actually did work for the circus, keeps that precious knowledge to himself:
'I used to carry water for the elephants,' says McGuinty.
[…] 'You did not,' I say. (1.40, 42)
It's funny that McGuinty is so quick to claim an experience that Jacob treasures and attempts to keep private. But when Jacob challenges him, McGuinty won't back down. He stubbornly has it in his head that he "used to carry water for the elephants." That's really all we ever learn about him.
Later, Rosemary tries to explain McGuinty to Jacob and to us, saying that he doesn't know the difference anymore between what really happened to him and what he thinks happened. It's hard not to wonder if Jacob's memory is doing the same thing to him. (Read all about it in the "Narrative Technique" section.) Jacob holds on pretty firmly to his stories and his past. McGuinty, it seems, is exactly the type of old man he doesn't want to be.