Janus is Virgil's 'Dark Side' or the opposite Side of Virgil. Here's why I think this:

1. Virgil's always the first to notice when Janus is pretending to be one of his friends when he's in the room. Virgil wasn't in the room when Janus pretended to be Logan (I think. I could be wrong)

2. Virgil's 'colouring' is black and purple and Janus' is black and yellow. Yellow and Purple are on the opposite sides of the colour wheel.

3. Virgil has always been truthful with Thomas, that's his job. His job is to tell Thomas the truth or what he thinks is the truth. Janus' job is to tell lies. Up until the newest Sanders Sides episode, he was know as Deceit which is another word for lies.

4. Janus is very confident. Virgil is the embodiment of anxiety. Anxiety and confidence are the opposite of each other.

5. Janus and Virgil hate each other the most. Janus hates most of the 'Light Sides' but he definitely hates Virgil the most. This could be because Virgil crossed over but, if we're going with my theory, it could also mean they're opposite sides.

Tell me what you think of my theory in the comments. Please?