Ruth is one of my very favourite single-syllable names. I'd use it happily as fn or mn. When we were about 6 or 7, a friend of mine at school whose name is
Ruth had trouble with some English sounds, and introduced herself serenely as Woof; not even that puts me off! I think
Ruth is classic rather than old-fashioned, and still very usable. I know a
Ruth Mary and her sister,
Anna Elizabeth; they are both in their 40s but could, I think, be any age at all.
Ruth CecilyRuth Felicity (the meanings are a bit much, but mns are seldom used so it'd be OK)
Harriet Betsy RuthRuth Joanna MaryRuth Penelope JeanRuth Philippa Esme