Dreamwalking

Dreamwalking is the act of entering another person's dream, usually through magical means. Many characters in the Super Mario series have displayed this ability.

Helena
Helena is by far the best example of a dreamwalker, although she is yet to appear in the Super Mario book series. She appears in the game that Super Mario Continuance (REITERATED) is based on, (MARIO) The Music Box -ARC-. She is the witch of illusion, and can enter the dreams of anybody she so chooses. In (MARIO) The Music Box -ARC-, she invades Mario's dream and tries to kill him, but fails. Mario instead kills her against his will.

Mario/Allen
Mario is initially shown to display the ability to dreamwalk in Mario & Luigi: Dream Team. In the game, it is innocent enough. He must travel through Luigi's dreams to save Princess Peach. This was, however, done through entering a direct portal in the waking world to Luigi's dream whilst Luigi was resting his head on a Pi'illo. So it may technically be considered a tool-assisted dreamwalk, and therefore not a legitimate dreamwalk. This could also be considered true for the Mario & Sonic series, where at night, the Olympians dream of the following day's Olympic Events, in something called "Dream Events". They all share the same dream through the magic blessed upon the Modern Olympics by the Ancient Olympians, and as such, enter each other's dreams.

Mario reprises this ability in some sense in Super Mario Continuance (REITERATED). Though this time, it is much more akin to how Helena uses it. The fourth Memory Fragment shows that Mario in the Allen iteration of himself, travels into Riba's dream when he still lived in Evangeline. Allen first appeared at the banquet table, but later appears on Riba's plate as only a head, and grows Alice's body. He then sends Riba falling down a tunnel, where he "falls out of the dream" and onto his bed, and then wakes up in the waking world. Lucas suggests that Birino Evangelisti is the one behind this, though the notion that Evangeline condemns magic, and seeing as how Birino is the head of the church disproves this. Also, Allen was wearing his iconic hat and overalls in the dream. Lucas soon drops this idea.

Allen, many centuries later, enters Riba's dream once more to direct him towards Abor's Cave. He pretends to be the iteration of Mario that Riba loved in order to trick him into going to the cave. Oddly enough, he wears a Tanooki Suit at the beginning of the dream.

Links and Resources
To learn more about dreamwalking in its more general use, go to this Wikipedia page: Oneironautics