At first, the demon you summoned was quiet and loyal, allowing you to do pretty much whatever you pleased. While he still does whatever you pleased with him, Sigma has changed a little.

Perhaps the demon was just showing it's true colors, but as he latched onto your arm and glared daggers at anyone who dared to even look at you it became a little obvious. Somehow, he was jealous. Jealous like a toxic partner.

Sigma held onto your arm to make sure no one got close to you, holding onto who he saw as his human. His and only his.