bind <character> self_insert
Insert the bound character into the input line at the cursor position. This may seem obvious – but every character needs to be bound to something, and for language characters, they are bound to SELF_INSERT.
If INSERT_MODE is ON, then the character is inserted “before” the cursor, and the cursor and everything after it is moved over one place.
If INSERT_MODE is OFF, then the character replaces the character under the cursor.