Nine has one of the more tragic arcs in "Sonic Prime," with a much different backstory than Tails. Unlike how Tails met Sonic in the original universe, Nine had no one growing up, as the Council took over New Yoke City, making him a distant and dark loner who trusts no one. Thus, when Sonic first meets Nine, he gets quite a cold shoulder until he is eventually able to get Nine on his side. However, after Sonic rejects Nine's request to go to a new world and start over, Nine clearly isn't happy and feels some of his old feelings resurfacing.
He is especially vexed when Sonic suddenly vanishes multiple times after touching the prism shard, leaving him feeling abandoned. So when he's talking with the Chaos Council about potentially working with them on improving their tech to traverse the Shatterverse, you can't help but wonder if Nine is actually planning on betraying Sonic. Perhaps this is just a momentary lapse: Nine's feelings of abandonment are undoubtedly real, but he's never seemed capable of outright villainy. Plus, any time that he's felt this way in the past, he's quickly made a turnaround to help Sonic, so it wouldn't be too surprising to see this happen again.
