The person you see on the roof is Yoshi.
THe reason for that, is in the story. Most of us know it, but I'll refresh your memories just in case.
Princess Peach invites Mario over for cake, and wants him to bring some friends, so he brings Wario and Luigi along. Yoshi got there too early for the cake, so he decided to go on the roof and take a nap. He awakens to find that the castle is deserted. Thinking he overslept and missed the cake, he goes to see if he can find everyone.
There you go. Yoshi is on the roof 'cause he wanted to take a nap.
I'm guessing Peach's voice sounds weird, 'cause they're trying to recreate the voice she had in the original SM64. I forgot who did her voice... I think it was Leslie Swan or something... I forgot her name. She was someone at NOA, 'cause originally, Peach didn't speak in the Japanese version.
Nothing but a Conservippie.