Can I make an enemy using the character builder rather than the monster builder, so basically you fight another character compared to a monster? I think it'll be easier for a part of the campaign I'm making. (kind of like a reoccurring nemesis which levels up every time you meet him, kinda like in the pokemon games)

I make all my important NPCs full character sheets. How I generate the stats can vary, as can if I stick to the RAW for giving it Feats, languages, et cetera, or if I just plug in all the things they need to fit the role. For NPCs that evolve throughout the story alongside the players, having a full sheet can be super helpful for scaling up their threat level (whether as an enemy or an ally). 

Thanks for the reply; I think having extra NPC "characters" will add some spice to the campaign, and maybe some humour. Who knows, I might even get someone extra to come in and play this nemesis I've planned for surprise.

I agree, having a "character" built using standard PC style rules for an NPC can serve as very interesting and engaging opposition with fun abilities and unique perspectives.

One thing to consider is that, especially if you are intending one of these antagonists to be mostly alone, balancing them as an encounter can be rather tricky.  Their actions available and HP amounts will always be low if they are a similar level to the characters, which means you will probably want them to work in a group (yay rival parties) or provide them with some minions in order to ensure an appropriate challenge for the PCs, and to give your antagonists a chance to not immediately be crushed.

Quote from ShadowPhage13 >>

I agree, having a "character" built using standard PC style rules for an NPC can serve as very interesting and engaging opposition with fun abilities and unique perspectives.

One thing to consider is that, especially if you are intending one of these antagonists to be mostly alone, balancing them as an encounter can be rather tricky.  Their actions available and HP amounts will always be low if they are a similar level to the characters, which means you will probably want them to work in a group (yay rival parties) or provide them with some minions in order to ensure an appropriate challenge for the PCs, and to give your antagonists a chance to not immediately be crushed.

 I totes agree with all of that. If the party is going to battle a solo NPC, I always have it be a much higher level. You can help balance the action by finding ways to give the NPC multiple attacks per round (available to some classes, or by just attacking more than once, you are the DM after all). I occasionally have to fudge some rolls and/or deus ex machina something to prevent a TPK, because I’m not trying to kill them, just show them a good time.

Or maybe the NPC challengers a party member to an honourable duel or something like that, and the party can decide to crash the duel for fun if they want. That'd be pretty funny to describe.