The things Salad Fingers says into the toilet and what the other Salad Fingers says to the first form at least part of a conversation when put together. When put together, it follows as:
First S.F.: Jeremy Fisher. I thought you were out fighting the great war.
Second S.F./Jeremy Fisher(?): Pardon?
First S.F.: I'd like a word with you.
Second S.F.: I don't understand. What is this?
First S.F.: I'm not one for accusations, but this one's CAST IRON. I've seen you tailgating my daughter with aspirations of deflowering her rose.
Second S.F.: No! This isn't true! You've got the wrong bloke, squire!
First S.F.: I don't make mistakes, comrade. This one's textbook.
Salad Fingers' fingers look how our hands would look without skin holding our fingers together. Go ahead and Google 'X-Rays Hands'. Doubles as a Fridge Horror.
However, Salad Fingers only has three fingers per hand, while we have five.
If going with the The End of the World as We Know It theory, then it might actually make sense, seeing that there are already people born with defects such as missing a finger on both hands. Radiation can cause mutations which could - in theory - make it possible for him to naturally have less fingers.
Alternatively, since we don't know how long that war had been ago, he might have lost fingers at some occurrence, though it would be questionable how he managed to lose exactly two fingers on both hands.
Salad tells Hubert that Glass Brother was raised under the "Beast's sun" and it should be worth noting that Salad couldn't pass through the mirror portals during the day. After Hubert is taken by Glass Brother and when he goes to get him back, Salad is able to go into the reflection world at night because Glass Brother wouldn't have been there to block his way.
Salad Fingers probably suffers from some severe dementia, psychosis or post traumatic stress disorder caused by "The Great War" he keeps referencing. He's constantly by himself, talking to finger puppets like they're his friends, and eating such things like the finger puppets, hair, and resorted to cannibalism.
He probably also developed his fetishes with pain, rusty metallic substances, and nettles, because that's all that's left.
Who do the hairs in episode 8 belong to?
Possibly Roger the Radio's wires...
We see five copies of Salad Fingers at the end of episode 10. There are five hairs. Roger, who claimed one of the hairs as its, resembles Glass Mother in its personality. Therefore, it's reasonable to assume that the five hairs belong to Fingers' five family members who used to live in his house before they died.
The ending of episode 5. The shock of hearing another normal voice after God knows how long drives poor Salad Fingers into a horrific Heroic BSoD, huddled in a corner of his room while hallucinating Mable's image, with empty, bloody eye-sockets, making more "mouth-words" at him. We never see her again after this. Now...why were her eyes gouged out like that? Was there a different kind of reactionthat we didn't see?
The plot of episode 11 seems to be a direct parallel to schizophrenia, where sufferers will have frequent auditory hallucinations, which are also frequently abusive to the person, forcing them to walk around all day while hearing torrents of abuse being hurled at them. In the episode, Salad Fingers believes his reflection is an abusive copy on the other side, and also sees his mother in the reflections, both of whom constantly torment him. In the end, he smashes his mirror, reducing his "glass mother" to a tiny shard he can barely hear, but even though he's tucked it away in a box where she won't be audible, it's still there.