I haven't seen the Japanese raw for this because it's nowhere to be found, but based on my understanding of the language and the way Kyouko typically speaks, I'm willing to bet that the part that Kyouko colorized in her text said 「すきです」. This phrase when taken out of context sounds identical to a love confession. Since in order to colorize it she basically separated the phrase from the context of the sentence, this connotation is hard to ignore. I mean, come on, it's literally flashing like a neon sign... Ren probably knows that Kyouko doesn't mean it that way, but he knows that Kijima is taking it as an open invitation to keep writing her to continue trying to win her over. It's no wonder Ren's angry... Kyouko's gonna get another lecture I think :) Though, as was demonstrated last time they fought about her actions in regard to Kijima, Ren is going to have a hard time finding a way to convince her she did something wrong without making his feelings known to her. Part of me thinks that with as angry as Ren is right now, and how difficult it's been to control his emotions lately, all he needs is one more thing to set him off where he'll just loose his cool and make some overt display to get other guys to back off and stop pursuing Kyouko. It's possible that finding out that Shou visited Kyouko at school will be his tipping point (since there's no way that's not happening next issue and that Ren will certainly find out). Then again, with how things have been going throughout the past year or longer, this plot will probably drag on, teasing us the whole time, until eventually nothing worthwhile happens.