It could be the one who is manipulating Rias. If there is an external force or character that is making Rias betray Naruto, then that entity would be the main antagonist. For example, a powerful and evil ninja who has a vendetta against Naruto might use Rias as a pawn to achieve his goals.
If Rias betrays Naruto, she herself could potentially be the main antagonist. Since Naruto is usually seen as a protagonist, Rias' actions of betrayal would put her in direct opposition to him. Her motives and actions would then drive the conflict in the story.
The best chapter pointer here is Chapter 25, "He's Actually Mr. Shen's Son?!". It opens with Therefore, from his childhood, Jiang Shixu had felt an extreme aversion to... and closes on Having worked beside Shen Lan for over a decade, she had been there when....
Chapter 644, "Has Shang Shaojing Actually Gone Bankrupt?!", is the clearest early answer because it turns Yinwan's situation into a concrete plot event.
Chapter 812, "You’re Extorting Money!", is the clearest early answer because it turns Meng Wanzhi's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Li actually called the police!
A practical answer is Chapter 17, because it turns the idea into a concrete scene. The chapter opens with Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and..., adds pressure through That girl who puts on makeup like a ghost deserves to be called naturally..., and ends on.
Chapter 815, "You've eaten through all the savings!", is the clearest early answer because it turns Grandpa's situation into a concrete plot event. It opens with Jiang Zhao Yuan was quite surprised, "Right, absolutely right, haha,...
Chapter 691, "Sorry, You Don't Deserve It", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event. The setup is As soon as Jiang Li was mentioned, Zhao Lang suddenly became..., but the pressure keeps building through How long have you been talking?.
Chapter 781, "Master, accept my bow!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Not only were the criminals curious, but so were the others. and closes on I really was chosen, I knew there was something wrong…" The audience....
Chapter 923, "Eight-legged essay?!", is the clearest early answer because it turns Miss Rong's situation into a concrete plot event. A useful checkpoint is the way the scene starts with I just went to her house for a meal, even if my relationship with my...
Chapter 763, "It must be Jiang Li who tied her up!", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. It opens with But within a short time, she sensed that something was off.
Look to Chapter 17 for the clearest answer. It begins with Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and..., keeps building through Okay, let's go eat." After speaking, she strode forward., and lands on I've been a mother for more than ten years but never tasted.