UndertaleThings's Guide to Gaster

Frisk - Go up, then go right to the bottom exit. Frisk will fall and "transform" into Chara, ending the character.

Chara - Go up, then go to the right of ball game. Chara will push the deer, "transform" into Frisk, then end the character.

Monster Kid - Go down into the Core, then go right. Monster Kid will trip and fall, ending the character.

idk who this girl is im sorry - Go right to the Snowdin Bridge, then Chara will introduce themselves and "lead them around Snowdin", ending the character.

Asriel - Go up, then to the left-down of Ball Game. Hug Frisk.

Gaster - At the end of X-ing out all of the characters, Gaster will appear. Simply mash Z and then stop the timer once Gaster appears.

UndertaleThingss