The Guide is the first NPC that the player will encounter in any given world. Unlike other NPCs, he does not need a house, but will move into the first one built, which will often be yours, or the first house you have explored; floating islands or the underground houses in giant trees. If the Guide dies before a house is built, he will not respawn until there is a suitable house in the world. Occasionally, the Guide will walk off cliffs or directly into caves without looking. Like other NPCs, he does not suffer any fall damage when that happens.
The Guide provides tips to new players in order to help them learn the basics of the game. After every available tip has been said, he will cycle through them again.
The Guide also gives all crafting recipes for any item that the player shows him via the "Crafting" option.
The Guide defends himself after the 1.3 update using a wooden bow and shoots stone arrows to nearby enemies dealing 10 damage each. This can be useful during the beginning of a world, as it will help to fend off zombies and other mobs.
"Greetings, <player name>. Is there something I can help you with?"
"I am here to give you advice on what to do next. It is recommended that you talk with me anytime you get stuck."
"They say there is a person who will tell you how to survive in this land... oh wait. That's me."
"You can tell a Blood Moon is out when the sky turns red. There is something about it that causes monsters to swarm."
"Hey, buddy, do you know where any deathweed is? Oh, no reason; just wondering, is all." (subtly suggesting that the player should gather Deathweed, because they only bloom and drop seeds during a blood moon)
"If you were to look up, you'd see that the moon is red right now."
"You can use your pickaxe to dig through dirt, and your axe to chop down trees. Just place your cursor over the tile and click!"
"If you want to survive, you will need to create weapons and shelter. Start by chopping down trees and gathering wood."
"Press ESC to access your crafting menu. When you have enough wood, create a workbench. This will allow you to create more complicated things, as long as you are standing close to it."
"You can build a shelter by placing wood or other blocks in the world. Don't forget to create and place walls."
"You should focus on gathering more heart crystals to increase your maximum life." (only when the player doesn't have a maximum health of 400)
Tips for first boss fight
When you have Iron or lower equipment: "Your current equipment simply won't do. You need to make better armor."
When you have silver or better equipment and haven't battled the Eye of Cthulhu yet: "I think you are ready for your first major battle. Gather some lenses from the eyeballs at night and take them to a demon altar."
When Eye of Cthulhu was already battled
"You will want to increase your life before facing your next challenge. Fifteen hearts should be enough."
"The ebonstone in the corruption can be purified using some powder from a dryad, or it can be destroyed with explosives."
"Your next step should be to explore the corrupt chasms. Find and destroy any shadow orb you find." (even though that may be a bad idea, since a new player following his tips will likely not know that once they destroy 3 Shadow Orbs, the Eater of Worlds will spawn even though they may not have fifteen hearts yet)
Tips for late Pre-Hardmode
"There is a old dungeon not far from here. Now would be a good time to go check it out."
"You should make an attempt to max out your available life. Try to gather twenty hearts."
"There are many treasures to be discovered in the jungle, if you are willing to dig deep enough."
"The underworld is made of a material called hellstone. It's perfect for making weapons and armor."
"When you are ready to challenge the keeper of the underworld, you will have to make a living sacrifice. Everything you need for it can be found in the underworld."
The Guide has a tendency to get stuck in caves, holes, etc. To avoid this, cover up any holes or caves within the Guide's area or just cover him up until you build a house for him.
In previous versions of the game, the Guide would wander around even at night, opening doors and letting hostile mobs in.
The Guide's hair seems to be a deformed version of Hair 2.
In the mobile version of Terraria, the Guide can no longer tell you crafting recipes. The recipes that include any or all of your items you have are displayed when you are crafting.
There is a painting in the game that appears to be the Guide called Guide Picasso.
The Guide will take damage from things like the Rainbow Gun if you equip the Guide Voodoo Doll after you cast the spell. If you remove the Doll, it will immediatly stop the damage.
The only time the Guide asks a question is on Blood Moons when he asks if the player has seen any Deathweed.
If the Guide's name is Andrew he will drop a Green Cap when killed, most likely a reference to Andrew being Redigit's real name.
Like any other NPC, the Guide won't respawn if there are any world events (like Goblin Attack) ongoing.
It's possible he will be named Guide the Guide instead of the names above.
If you select the "Crafting" button while standing near a crafting surface and there's a lit up item, you can craft that item from the Guide.
Sometimes the Guide will say "Hang on to any bombs you might find. A Demolitionist may want to to take a look at them" when the Demolitionist is already present.
Sometimes the Guide might not spawn back too fast or never.
When the hunter potion is used the Guide will light up, It appears that the Merchant is the only pre-hard mode NPC that won't light up.
Even thought your world has been summoned with The Crimson instead of The Corruption, the Guide will keep telling you to destroy "Shadow Orbs" on the Corruption, despite having destroyed many Pulsating Hearts.
If your world is Crimson the Guide will still say "Make sure to smash any Demon Altar you can find. Something good is bound to happen if you do!"