How to use UNICODE characters in PL/SQL?

We had a requirement to write a pl/sql function to return strings in sinhala/tamil characters. So just copying and pasting the sinhala/tamil characters in the function body don't work. Hence we need to use unicode characters for this. just using unicode characters not sufficient. Should use the
UNISTR function.
Eg:
UNISTR('\0DB1\0DDC:') returns අංක: (Sinhala)
UNISTR('\0B87\0BB2:') returns இல: (Tamil)

Although the stored unicode characters values be taken as
ASCIISTR(column_name)