The stranger who stayed with us was a ruthless fellow.

Answer: Option B

Ruthless : having or showing no pity or compassion for others.
Candid : truthful and straightforward; frank.
Sympathetic : feeling, showing, or expressing sympathy.
Cordial : warm and friendly.
Courteous : polite, respectful, or considerate in manner.

Antonym of Ruthless is Sympathetic