A likely main character could be a scientist from Earth who is excited about the opportunity to study the advanced technology of the Star Wars universe. They would be in the middle of the relationship between Earth and the CIS, trying to use the alliance for scientific progress. Also, a CIS spy on Earth could be a main character, tasked with ensuring Earth's loyalty and reporting any signs of betrayal.
The main character could be a young and idealistic Earthling who believes that joining the CIS will bring a new era of peace and prosperity to Earth. They volunteer to be part of the first Earth - CIS joint military unit. Alongside them, there could be a jaded CIS politician who has seen many alliances fail and is initially against Earth joining but is overruled. And a Jedi who senses the imbalance that Earth's joining might bring and tries to interfere in the situation.