My theory on L's past

Orphanage.

So what we also believe is that he was in the orphanage from the age of 1-16 at the age 16 he got out.
In the orphanage he often stayed in his room and read.
He also studied and got the best grade in the orphanage (they had a sort of school like thing in the orphanage) and the way he sits is something he was told by one of the workers in the orphanage and he has always thought it was true.
We also believe it explains his pale skin, we believe he would hide inside and read, study and research.
And his eye bags showing lack of sleep we believe he was kept up by the younger children and and stayed up reading and losing track of time, and he just never grew out of this habit.
His friendships/ lack of friendship. He would often hide in his room so he was use to being isolated/ hidden.