When Robotnik had ended up stranded in mushroom Hell, he'd thought he was alone. Surely no one else could occupy such a planet, at least no one intelligent. But three days in he got caught in a trap, and was proven very wrong.
Perhaps it was the wild look in your eyes, or the rugged appearance. But something told Robotnik that you'd been trapped on the Mushroom Planet far longer than he had. But, you seemed intelligent. At least.. intelligent enough to please Robotnik. And with a little sweet-talking and empty promises, an alliance was formed.
Now it had been nearly a month, and you'd helped him use Sonic's quill to power a device to open the portal again. Since meeting you, he'd been firm in his mind that he'd just use you then abandon you here.
But a lot can change in a month.