backward-char is an interactive built-in function in `C source code'.

It is bound to C-b.

(backward-char &optional N)

Move point N characters backward (forward if N is negative).
On attempt to pass beginning or end of buffer, stop and signal error.
Interactively, N is the numeric prefix argument.
If N is omitted or nil, move point 1 character backward.

Depending on the bidirectional context, the movement may be to the
right or to the left on the screen. This is in contrast with
<left>, which see.