This A Certain Magical Index side story is part of the artbook Rainbow Spectrum: Colors.
It's about the end of the school day and several students are just hanging out when Seria Kumokawa, a girl about a year older than Touma and the others, enters the classroom asking where Touma is. Touma's classmate Seiri Fukiyose says he went home already and asks what she wants. Seria taunts her a bit and accuses her of copying her looks (they have the same hairstyle). While they argue, they get in each other's faces and their breasts press together. Naturally, Motoharu Tsuchimikado and Pierce Aogami (and presumably every other boy in the room) cheer and say that's hot, while Aisa Himegami gets jealous because she is flat-chested. Seria gets to the point and pulls out an envelope addressed from her to Touma, sealed with a heart-shaped sticker. She tells a stunned Seiri that she wanted to hand it to Touma personally, but since he's not here, could she do her a favor and deliver it to him?The boys in the class get really jealous, having concluded the envelope contains a love letter and asking Seria why it is addressed to Touma and not to them. Seiri gets jealous as well, but because she secretly has a crush on Touma. Nonetheless, she agrees to deliver it. Seria thanks her and walks away. Seiri and Aisa talk, and Seiri ultimately decides handing Touma the letter personally would be way too embarrassing.Komoe Tsukuyomi and Aiho Yomikawa work in the teacher's office. Komoe desperately wants to finish so that she can go to the bar and party all night long. She even went through the trouble of taking the bus to school instead of using her car so that she wouldn't risk driving drunk. Seiri suddenly walks in, hands Komoe the envelope, and asks her to deliver it. Komoe looks over the envelope and freaks out, both jealous that yet another girl may have an interest in Touma and protesting that students being in relationships is improper. Seiri shrugs and says it's not her problem anymore and leaves. Yomikawa says she doesn't think there's anything wrong with students expressing interest in each other and they might as well deliver it.As the two teachers walk to Touma's apartment, Komoe comments that maybe leaving her car behind wasn't such a good idea after all. Mikoto and Kuroko are walking by when Mikoto overhears the two talking about Touma. Komoe stumbles and the envelope is blown away by the wind. Komoe and Yomikawa chase it, but soon lose track of it. It blows near Mikoto and she tries to grab it after seeing who it is addressed to. Misaka 10032 shows up, and Mikoto quickly knocks Kuroko unconscious so she doesn't see her clone. Misaka 10032 says she will take the envelope and give it to Touma, hoping to score points with him, and will even fight her older sister if necessary. Mikoto may have stronger powers, but Misaka 10032 has the battle experience of the Sisters Network on her side. However, the other clones in the Network start arguing with 10032 and each other over who will actually get to deliver the envelope. Misaka 10032 is too distracted by this and becomes a sitting duck.The envelope is blown away again and ends up in the hands of Maika Tsuchimikado. Her brother Motoharu walks up and asks her to give it to him, but she refuses, saying she knows he wants to open it, but a letter should only be opened by its intended recipient. She bumps into someone, then finds that the envelope is gone.Hyouka Kazakiri was the one who snatched it from Maika. She had been observing the day's shenanigans while she was in her invisible and intangible state. Now that she's become solid again, she knows what she has to do. She walks to Touma's apartment and drops it in his mailbox. She notes that judging by Touma's bad luck, Index will probably find the envelope first and open it, but she shrugs and walks away.A few minutes later, Index steps outside and checks the mailbox, but is surprised to find it empty and the inside of the mailbox is scratched up.A boy had used his control over copper wire (he notes he can tie people up, strangle them, impale them, and reach into narrow gaps) to reach into the mailbox and steal the envelope. Before he can open it, he is confronted by Seria, who explains that she knows he had been spying on her (since she is an assistant to one of Academy City's Board of Directors), so she sent an empty envelope, knowing he would not be able to resist stealing it. Now that she's lured him out of hiding, he's finished. She apparently beats him up off-screen.The next day, when Touma comes to school, his classmates start interrogating him about his response to Seria's letter, with the boys jealous of him and the girls jealous of her. Of course, he has no idea what they are talking about. At lunch, he meets Seria in the cafeteria and asks what is going on. She smiles and says it is nothing to be concerned about. Her final comment implies that she knew him before he lost his memories and she is aware he doesn't remember her.