We see the word "Ruthless" used (mostly to mean merciless or cruel, like a tyrant) but we rarely see "Ruthful".
Also, it's interesting that thesauri mainly list Sympathetic and Compassionate as antonyms of Ruthless. Likewise, Ruthless is generally not listed as the main antonym of Ruthful.
Guess it's just one of those things where usage sense and frequency has drifted w time. Nothing to rue though. 😉
The actress (Zoe Perry) that plays Mary Cooper on Young Sheldon is the real-life daughter of the actress (Laurie Metcalf) that played the same character, Mary Cooper, on The Big Bang Theory.