The Guide is the first encountered NPC in the game. He offers tips to aid survival and crafting help for the new Terraria players. He will also occupy the first built house.
 To remove the Guide
Many players dislike the Guide because he can be a nuisance by opening doors and not closing them. This can allow various sorts of Monsters to enter the player's home. You can avoid this by building him a home of his own or by trapping him in a hole/room. Although, if a house is built with the necessary requirements for an NPC, he will move into it when the player is off-screen for a certain amount of time (whether he is trapped or not.) If these two options aren't for you, then it is possible to kill him with lava or by using the Guide Voodoo Doll. However he will respawn afterwards.
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