And -

Like two sides of a similar coin

Positive and Negative are both two different iterations of the skecthpad that was made into an SCP that then branched Simon off of it.  Originally, the sentient sketchpad would communicate telekinetically.  Soon it grew tired and came to take a form easier to cope with.  Unfortunately, only Simon can seen either of it.

Basic Information (as given by Simon)

Positive is the manic, yet friendlier, side of SCP-3000.  Her general appearance is friendly as she is (to SCP-3000-1A) but is actually psychotic in nature and is best to stay away from when she is "spacing out".  This nature does not intimidate SCP-3000-1A, and is known to seem like she tries to, but both the sketchpad and Simon know they rely on each other enough to keep to themselves.  

Negative is the passive-agressive, calm side to SCP-3000.  Compared to Positive, Negative is very collected and introverted in nature.  Simon claims that it is unexpectedly easier to keep a conversation with Negative, but not an interesting conversation nonetheless.  He once asked Negative how his suit was torn to the reply was, "If you believe me, it's your suit.  It's two sides of the same coin.  The clean, wool side above the ripped, linen side".

Both "versions" of the sentient sketchpad don't ever seem to transform to what they are.  Simon claims that one of them at a time roams around whatever general area he is in, but only after noticing the sketchpad itself is missing from him.  This is because, as he says, they themselves are the corporeal form of the sketchpad.  He only seems to ever speak to them when they ever want to speak to him, making it hard to predict and plan a desired conversation.