Count of all character, one per line
awk '{for(i=1;i<=NF;i++)if(!a[$i]++)print $i}' FS= *.xml
Count of all character, same line
awk '{for(i=1;i<=NF;i++)if(!a[$i]++)print $i}' ORS= FS= *.xml

Technical notes, and other ideas.
Count of all character, one per line
awk '{for(i=1;i<=NF;i++)if(!a[$i]++)print $i}' FS= *.xml
Count of all character, same line
awk '{for(i=1;i<=NF;i++)if(!a[$i]++)print $i}' ORS= FS= *.xml