In the flashbacks, it's shown that Degwin Zabi increasingly notices the similarities Gihren has with the late Zeon Deikun, to the point of calling him a fiend possessed by Deikun's grudges. It's no surprise then how he managed to win over Deikun's loyalists over to his side. And even Char himself.
During the flashback, it's shown that Char is the reason Dozle met Zenna and fell for her, and he knows it. Suddenly, his favoritism for Char until Garma's death makes sense: this guy is the one reason he has a wife and daughter.
Before leaving for the Military Academy, the real Char Aznable expressed his pro-Zabi views right in front of Édouard Mass during dinner. You could see the wheels turning in his mind realizing that as a Zabi loyalist, he will have to eliminate him before he would become a threat in the future. And that's not even mentioning the ramifications of someone who could be a double for Casval Rem Deikun being a Zabi supporter.
