Shii

    ∧ ∧___  ダッコ♪
   /(*゚ー゚) /\
 /| ̄∪∪ ̄|\/
   |        |/
     ̄ ̄ ̄ ̄

Shii (しぃ) is a stray cat that lives in a box. She is Giko's girlfriend. Shii is a part of the Big Four, but is less commonly used nowadays.

Shii is a common victim in abuse threads, where she is abused by Moralar.

       ∧_∧  
       (*゚ー゚) ____
        /つ/|     /| 
     ~   | ̄ ̄ ̄ ̄| .| 
    、、、、(/  | ギコニャン. |/,,,  ,,

Afoshii

   ∧_/∧ ∧
  ( ・∀(゚ー゚*)
  ( つ と  0
    )  ⊂ _,ノ~
  (__)_)   

The version of Shii that shows up in abuse threads is sometimes referred to as "Afoshii" (アフォしぃ) meaning "Idiot Shii", to distinguish it from the regular version Shii. Afoshiis are generally more selfish and annoying than regular Shiis.

This copypasta is often used to explain what Afoshii is like:

Afoshii "Real" Shii
Often travel in groups. Even if you crush them, they will continue to flourish. It is rare to see two or more.
Blatantly uses phrases such as "Hanyan", "Matari" (relaxing/chilling), "Dakko" (hugs), "Koubi" (sex), etc. She'd like to talk about those things, but she keeps it to herself.
She thinks she's cuter than anyone else. People around her say she's cute, but she doesn't act that way at all.
Uses long sentences and loose reasoning. Reticent.
Her emotions are exposed, and her facial expressions change constantly. Her expression is as cool as Giko's, and I can't read her thoughts.
The shape of her butt is strange. Thinner than Giko.
Begs to be hugged/carried. No such thing.
Is blatantly willing to commit abuse for the sake of Dakko, Matari, and Hanyan. No such thing.
Tries to solve everything with Dakko. Tries to solve things logically.

   ∧ ∧ キィ!キィィ!
   (*゚ー゚)      ∧∧
   U  |   ∧,,,,,,(゚ー゚*)
 ~(  _,つΣ※<#と  )
   U´   (/"(/`;;(_つ_`⊃

Basically, it's a version of Shii that has been changed to make it more satisfying to torture. However, like all abuse-related characters, Afoshii is controversial. Fans of Shii say it corrupts her character as a shitty attempt to justify her abuse.

Afoshii is also sometimes called "rag bug" (雑巾虫) or "shitty doujin insect cat" (糞同人虫猫).

Sources: