I have been following this discussion for awhile and i have to say Yakumo's character has been misconstrued and off the mark. Yakumo's character has been consistant and the slap isnt something out of character for her. She is shy, introverted person, but she will protect those people she loves. Yakumo doesnt have to redeem herself, and i dont see anything of the sort happening. Both Eri and her will probably act as if it didnt happen.
In truth, the slap incident occured through Eri's own fault. Eri, for the lack of a better word, belittled Tenma, what was Yakumo suppose to do? Tell her to stop, no. Eri's character wouldnt have listened, the only people who have that kind of power over Eri are Harima and Akira, and maybe miko-chan. Eri is a feisty girl, thats what i love about her, but feisty girls dont listen. And dont put feisty girls on a pedalstool. I'm sorry, yakumo did what she had to do, she protected her sister. I'm not saying she is a 'good girl," but we know who she is as a full fledged character.
i'm not condoning violence, it can be a childish act, there are times when you have to use it to protect someone. I'm not showing bias, cause well i'm a Flag supporter, and Eri deserved it. It allowed her to put what she thinks and what she says in a consistant manner. We know Eri is a caring person, but it is not our thoughts but our actions that define us a person. Eri's actions were off with how she felt when first confronting tenma and after the slap she regained perspective again. Which gave us chapter 212.