Answer

http://www.pixiv.net/member_illust.php?mode=medium&illust_id=53706230

Cannot name all of them need help

Tried to add some based on my knowledge and answer but still far from complete.

I've added quite a few more characters to the list.

The character who was labelled as Yin (Darker than Black isn't even a light novel series...) is actually Tokitsuki Yukino from Danshou no Grim, so I fixed that as well.

still missing:
*the girl facing to the left in the top right
*the short haired girl up left from Haruhi
*the girl down left from fremy
*the two between Victorique, Kino and Tohka (I should know the lower of the two...)
*the one with her eyes closed facing to the left and holding a book
*the small one with the glasses next to her
*the one on the far left, to the left of yozora
*the tiny one in the bottom left corner
*the two along the bottom to the right of Nyaruko
*the short haired girl above them
*the girl with heterochromia above her who I should so know and am currently getting really irritated with myself for not getting
*the one very much in the background between her, Kurousagi's arm and Yokinoshita's leg
*the girl between Yukinoshita's legs (that sounds really lewd but there we go)
*the girl above Hestia's ponytail who could be any one of a thousand characters and probably won't be confirmed without the artist themself clarifying.
*the two girls between Yukinoshita and Mikoto
*the girl between Mikoto and Tooko (I should know this one too)
*the girl to the right of Tooko
*the one with long blonde hair to the left of Megumi
*the tiny one with braided hair up right from Megumi
*the other tiny one with braided hair in the bottom right corner
*any others that I've completely failed to notice.

I may come back to this at some point.

edit: just saw the "answer" link... lol. Will go through the rest tomorrow if noone else does before that.

Still missing:
*the girl facing to the left in the top right done
*the tiny one in the bottom left corner - known but how to add that series' name to tags?
*the girl above Hestia's ponytail - done.
*the girl with flowers in her hair that Yukinoshita seems to be sitting on - now added

Character names needed for:
Double Brid (the one in the bottom middle between Lillia and Momo) - added
Tobira no Soto (the one up right from Megumi) - added

Began to annoted some characters.

There's one I don't know so I had to search for it but not so many info about Rengoku Hime.
But my guess is she's Artemisia Palo Lae (アルテミシア=パロ=ラエ).

EDIT:
I think I found another character, from Coffee-Ten Talleyrand no Jikenbo: Kirima Mihoshi (切間美星 (きりま みほし)).
And Saotome Aoi (左乙女 葵 (さおとめ あおい)) from Hikaru ga Chikyuu ga Itakoro.
Oh wait, they were tagged already but no idea who they were.

Everyone with their character name in the tags has a note now. Still the five mentioned above missing.

[deleted]

[deleted]

It took me quite a lot of research but I think that the character from Double Brid (between Lillia and Momo) is Katakura Yuuki (片倉優樹 (かたくら ゆうき)).
In Google pic, it show me a girl with short hair but in Pixiv, it show me both long and short haired.

EDIT: As for Tobira no Soto, looks like she's Izumi Reiko (和泉玲子 (いずみれいこ)).
Not much info about her but there's one fanart of her on Pixiv which is the only one who has a single braid.

Man, characters from Light Novels are always hard to identify... Annotation not added BTW. I thought it needed some confirmation.

EDIT 2: Think I found the girl below Yukinoshita Yukino.
It's from a Light Novel called "Mahou Shoujo Ikusei Keikaku" (魔法少女育成計画 (まほうしょうじょいくせいけいかく)).
Her name as a human is Himekawa Koyuki (姫河小雪 (ひめかわこゆき)), and as a Magical Girl (the one on the picture here), Snow White (スノーホワイト).
Really had a hard time to find because of the handwritting from the pic answer.

EDIT 3: Found the one near Celty Sturluson.
Her name is Yaku (ヤク) from Nisen Yon Hyaku Kyuu (or 2409) Kai no Kanojo (二四〇九階の彼女 (にせんよんひゃくきゅうかいのかのじょ)).

EDIT 4: The last one is tricky, seems the beginning is written Kaidan (階段) but can't recognize the last kanji.
It confuse me just like the Mahou Shoujo Ikusei one where the first kanji is written differently.

Confirmed the two character names via a combination of pixiv, google search and the original light novels. Confirmed Snow White from Mahou Shoujo Ikusei Keikaku thanks to someone who is reading the series. The other two are still yet to add. And no I can't read that kanji either.

I think I'll give up for the last one.
No matter how I try to retrace the kanji to identify it, I can't find something concrete.
It's kinda frustrating when you had almost everything except one...

She's Manabe Yu (真辺由宇) from Kaidantou Series (階段島シリーズ). Kudos to Zefyris for working it out.

kuuderes_shadow said:

She's Manabe Yu (真辺由宇) from Kaidantou Series (階段島シリーズ). Kudos to Zefyris for working it out.

So it was 島...
Damn handwritting...
Well, at least every characters has been identified now.
Only need a tag for 2409 Kai no Kanojo then.

EDIT 3: Found the one near Celty Sturluson.
Her name is Yaku (ヤク) from Nisen Yon Hyaku Kyuu (or 2409) Kai no Kanojo (二四〇九階の彼女 (にせんよんひゃくきゅうかいのかのじょ)).

2409 Kai no Kanojo Reader here, I can confirm that she is not Yaku. She doesn't have red bow and hairband. She is definitely The girl at 2409th floor (yes, that's what they called her.)

James_Liberty said:

2409 Kai no Kanojo Reader here, I can confirm that she is not Yaku. She doesn't have red bow and hairband. She is definitely The girl at 2409th floor (yes, that's what they called her.)

It's been a long time, I don't remember much. I believe They didn't have much illustration back then.
I think it was on the wiki page after I typed the name of the series.
Having no idea which character was it, I just searched the name of the series + the girl name.
And I found the exact girl when I typed it with Yaku, so I thought it was her.

user_441999 said:

I've added quite a few more characters to the list.

The character who was labelled as Yin (Darker than Black isn't even a light novel series...) is actually Tokitsuki Yukino from Danshou no Grim, so I fixed that as well.

still missing:
*the girl facing to the left in the top right
*the short haired girl up left from Haruhi
*the girl down left from fremy
*the two between Victorique, Kino and Tohka (I should know the lower of the two...)
*the one with her eyes closed facing to the left and holding a book
*the small one with the glasses next to her
*the one on the far left, to the left of yozora
*the tiny one in the bottom left corner
*the two along the bottom to the right of Nyaruko
*the short haired girl above them
*the girl with heterochromia above her who I should so know and am currently getting really irritated with myself for not getting
*the one very much in the background between her, Kurousagi's arm and Yokinoshita's leg
*the girl between Yukinoshita's legs (that sounds really lewd but there we go)
*the girl above Hestia's ponytail who could be any one of a thousand characters and probably won't be confirmed without the artist themself clarifying.
*the two girls between Yukinoshita and Mikoto
*the girl between Mikoto and Tooko (I should know this one too)
*the girl to the right of Tooko
*the one with long blonde hair to the left of Megumi
*the tiny one with braided hair up right from Megumi
*the other tiny one with braided hair in the bottom right corner
*any others that I've completely failed to notice.

I may come back to this at some point.

edit: just saw the "answer" link... lol. Will go through the rest tomorrow if noone else does before that.

The one with Red hair and Katana is Shana from Shakugan no Shana.