[SPOILER="Foreshadowing"]Oda Nobunaga was betrayed in the Honnouji incident when he went to go help one of his Generals who was being pushed back. His general who stayed at home came out of nowhere and killed him. Then General nicknamed Monkey then avenged Nobunaga! Sanageyama uses the kanji for monkey.[/SPOILER]
I wonder.
[SPOILER="About Foreshadowing and my speculations"]You say someone may betray Satsuki. Yet who? This show is so unpredictable with that part of the battle in Osaka and not Kyoto. I'm pretty sure they are trying to make not-too-similar parallels to Nobunaga with this show from the very beginning, with main setting in Honnoji Academy, which was leading Satsuki towards a fall from grace and the end of her ambitions, yet not towards her death.
Nui, who is unpredictable and extremely cruel, has the highest chance to be the betrayer. After all, she is one of the servants to the Kiryuin family. The one who betrayed Nobunaga had a history of rising towards power from a lowly position and had many names, so I feel as though Nui's disguise and charm was foreshadowing her true role in the series. It is also heavily implied that Satsuki would have trouble conquering the tri-city schools if not for the doings of Nui, which includes cutting Senketsu into several pieces and so allowing her to distribute out the pieces to power up the student army. Also, being the one who designs clothes (Grand Couturier), possibly for Revocs, and being able to manipulate life fibres, she's really the one with power and not the Kiryuins. She might even be the one manipulating Ragyo.
If it's not Nui, it could also be Shiro Iori, the guy who researches Life Fibre Uniforms. He doesn't present himself very much (but has near General status) so he's very likely too.
Another betrayal candidate could also be her own mother (Ragyo), one of the least likely people to appear. Might be involved together with Nui.
Matoi Ryuko is extremely unlikely since she isn't a general and was never on the side of Satsuki. Yet, since she was one of those not expected to appear and wishes to claim back Senketsu, she might have a part to play in Satsuki's downfall, and/or she might also save her because she still needs to know the answers to her father's death.
I feel they are using Houka Inumuta as a red herring, but that doesn't make him less likely to be a betrayer, since he greatly values information and won't fight to the death for Satsuki.
Nonon Jakuzure participated in one of the battles, so it is unlikely she will be the one to betray.
Monkey and toad men may well be the most loyal to her, so nothing to say about them unless the writers decide to do other unpredictable twists to the story.
[/SPOILER]