Geno (whose real name is the unpronounceable ♥♪!?) is a warrior from Star Road and one of Mario's partners in Super Mario RPG: Legend of Seven Stars. He is sent by "a higher authority" to restore peace to Mario's world. Although he takes the form of a wooden doll, Geno is presumably a star being.
When Smithy launches his attack on Mushroom World, he decides to make Bowser's Keep his base of operations. Thus Exor, the dimensional gateway to Smithy's world, crashes into the castle and occupies it. However, as Exor fell from the sky, he shatters the Star Road. As a result, seven Star Pieces are scattered throughout Mushroom World. With the Star Road broken, wishes cannot be granted. Because of this, Geno is sent to retrieve the Star Pieces and restore the power of wishes to the world.
During this time of crisis, Mario and Mallow arrive in Rose Town. Seeking shelter from the arrows, the pair enters the town inn. Inside, Mario sees a young boy named Gaz playing with his dolls of Mario, Peach, and Bowser. He is recreating one of Mario's various adventures. Gaz's mom, who is also the innkeeper, asked the plumber to play with her son. Gaz pretends to be Mario, and Mario pretends to be Bowser. During the pretend battle, the Mario Doll "bought the farm," so Gaz brings his newest doll: Geno. However, Gaz plays a little too rough. He activated Geno's "Super Star Shot" and instead of hitting the Bowser Doll, the blast inadvertently hits Mario, knocking him out.
That night, a single star being, the one later known as Geno, enters the Inn. Seeing the four dolls on the floor, the star decides to inhabit the Geno doll. Later, Geno admits he chose that particular doll because it appears to be the strongest of the four.
The next morning, Gaz tells his mom that he saw a living Geno walk into the Forest Maze. Because of this, Gaz receives a time out from his mother for telling a lie (which, of course, isn't). Mario believes the kid's tale and enters the woods to find the animated doll. Eventually, Mario catches up with him. Geno enters the forest to challenge Bowyer, the arrow-firing fiend, and retrieves the Star Piece that Bowyer discovers. Mario and Mallow decide to help Geno, and together, the three defeat Bowyer and acquire the Star Piece.
Eventually, Mario, Mallow, Geno, Bowser, and Toadstool collect the remaining Star Pieces and defeat the maniacal Smithy. With Smithy destroyed, Geno decides to leave the doll's wooden body, returning to the form of a star and bringing the Star Pieces to Star Road. Repairing the celestial road, the power of wishes return to the world. The lifeless Geno doll can be seen sitting behind Mario and company on Vista Hill while they witness Exor disintegrate. It is reasonable to assume that it is returned to Gaz, since Geno promises to return to Gaz after his adventure ends. (However, as elaborated below, Mario & Luigi: SuperStar Saga suggests that the doll later got a "job" in Little Fungitown.)