As this is an original story, in case someone stumbles upon this unfamiliar with Japanese terminology, I'll give the most direct translation of words used here (which is why this is at the end of the book, since that's usually where this sort of thing is put). It will be updated as I use the words, since I dislike not using words that have a lot more meaning in Japanese than they do when translated into English. Below that will be character reference images.

Aniki -- older brother

"-chan" -- most commonly used for girls, it's an honorific of affection (although it can be used to poke fun as well). For example, saying "Sam-chan" would be like saying "Sammy." Generally it's to addres friend-to-friend, lovers, or young children. 

Characters -- 

The Matsumoto brothers

1. Touma

2. Asahi

3. Itsuki

Note-- his eyes are the opposite of Touma's (left red, right gold), but there was no way to find a picture for that. Also, despite the stark difference in the pictures, he and Touma are identical twins. Just keep that in mind when imagining them. 

Also Itsuki--

The Sasaki brothers

Kyouya

Kouta

Others

Koizuma Hitamu

And finally, if you want to have a mental image of our heroine (which, since it's second person you don't really need), you have...

Himari