I was contemplating my incoming niece, for whom this post is named, when I chanced upon the word "ruthless". Now, obviously, if one can be ruthless one must also be able to be ruthful. I investigated.
Apparently, ruthless is actually a conjugation of "rue". Thus someone who does not rue, does no ruing, and has rued, rues and will rue nothing, is not a ruer but is ruthless (and not rueless). However, a practicing ruer can be said to be either rueful or ruthful, have ruefulness or ruthfulness, and speak ruefully or ruthfully.
This is your nerdy grammar lesson for the day.