Head Proportion Mechanics
Similar to the mushroom system of the various Mario games, Marisa can grow between various powers that affects the way she interacts with the stages.
The various levels are:
2 -> 3 -> 4 -> 6 -> 8
2: You start here at the beginning of each stage, regardless of the previous stage. One hit at this level and you're dead. 3: Marisa is a little bigger. She can take one hit and shrink down to 2, but aside from that and the size difference, there is no functional difference. 4: Marisa is the same size as head proportion 3, but is now able to break bricks. 6: Marisa is able to shoot fireballs straight forward. 8: Similar to a star in Mario games, she is invincible. While invincible, Marisa naturally cannot be hurt, and kills any enemy she touches (which combos for the duration of her invincibility). She can also destroy any normal block just by walking through it (note that she can still stand on them). After the invincibility runs out, however, she returns to the head proportion 4. If you're able to grab more red mushrooms (see below), the invincibility time stacks (ref: Extra stage 11)
Small (brown) mushrooms increase head proportion by 1, to a limit of 4. Larger (red) mushrooms double Marisa's head proportion to 4, 6, or 8.
If a proportion 6 Marisa is hit by an enemy/projectile, she shrinks to 3; 4 or 3 shrinks to 2; and at 2, dies, as mentioned above.
With Super Marisa Land patch v1.10 and higher, Alice is an available character - but she does play rather differently. For starters, she is limited to proportion 2 (any hit, as well as any mushroom, will kill her). She does get a "thrown" projectile - proportion 6 Marisa.