With few exceptions, all weapons and accessories (but not armor) will often have random "modifiers" applied when they're generated, crafted, or purchased. These modifiers can be positive or negative stat adjustments. There are a finite number of possible modifier sets that can be assigned to a given type of item. Each distinct modifier set confers a unique prefix to the name of the item in question, such as "Rapid," "Sharp," "Powerful," or "Keen." The full list of all possible modifiers is far too lengthy to include here in full, but it can be found on the official wiki.
Once you obtain access to the Goblin Tinkerer NPC, he will provide you with an extremely valuable service: reforging. Via reforging, you can pay some money to "reroll" the modifiers on a piece of equipment, by replacing the item's current modifiers with a new random set from the above list. If you seek to maximize your equipment's effectiveness by trying to get strong modifiers, reforging is an excellent way to do so, though it can be quite expensive to reforge higher-quality items.
When the Goblin Tinkerer reforges an item, it will always have a modifier. It is not possible for the Tinkerer to reforge an item and leave it without a modifier.
A common technique to beef up a character's toughness is to reforge most or all of his/her accessories to +3/+4 Defense.
Generally speaking, the best modifier for melee weapons is Legendary, the best for ranged weapons is Unreal, and the best for magic weapons is Mythical. Some exceptions exist, however; for example, modifiers that enhance weapon knockback aren't possible to obtain on weapons that innately have zero knockback.
Best Answer
NPC names are saved as text in the world file. I'm not aware of any save editor that can rename NPCs at this point.
However: There are currently* 35 possible names for the Guide, and — one of them is, in fact, Brian. Since the name is completely random each time, there's no telling how many of the gentle Guides you'll have to mercilessly eviscerate before finally getting a Brian.
Although, at about the 24th slain Guide, your chances of having gotten at least one Brian will approach 50%.
Hex editing
If you really want to rename your NPCs badly, you can do so by editing your raw save file with a hex editor like HxD. This should go without saying, but make a backup first because it's very easy to fry your save file.
Currently*, the NPC names are at the very end of the save file. They're saved as strings with length prefixes, so edit the name to be whatever you want, then edit the byte before it to reflect the length. For example, "Brian" has 5 characters, so you'd edit the preceding character to be
\x05(ENQ).The maximum length for NPC names is 22, and obviously they will reset if the NPC dies.
* Terraria 1.1