Cass sounds quite masculine to me so I'd prefer nn
Cass on a boy than hear it as a nn for something like
Cassandra. I like
Caspian (my favourite!),
Cassian and
Cassius.
Casper reminds me of the ghost too much.
Casimir looks and sounds handsome but I don't really like its negative meaning. I like
Johnny Cash but the name
Cash doesn't appeal to me at all, sorry. Suki doesn't appeal to me so much that I'd consider using it but its nice enough for someone else. I think I'd prefer it as a nn, though.