The relative pronouns are nominative and accusative a, oblique cases ydd, yr, y.
Each of the personal pronouns (except the 3rd plur.) exists in a longer and a shorter form: the one is used as a nominative and is a separate word, the other is attached to verbs and (in a slightly different form) to nouns to express the accusative or genitive.
The accusative is often the case represented in Welsh; but we have also the nominative, and sometimes both, as in cawed from civit-as, and ciwdod from civitat-em, now two words, not two cases of the same word.
In the meantime we have proper names to argue from; and these give us at least the significant indication that the Hittite nominative ended in s and the accusative in m.