The aid-giver!
When Mary's mother felt that things were going wrong, she wrote letters out to all the potential aid-givers, and those who she requested aid from naturally fit the criteria of an acquaintance.
Galeart was obviously suspecting that Mary's mother's death was related to these aid-givers, which explained his attitude towards Kieran.
Kieran was not mad at all. Instead, he wanted to thank Galeart for his vigilant attitude that made him speculate more.
It was a fact that Mary's mother was not weak, and those she requested aid from, how weak could they be?
However, there were two bodies hanging on the rail outside Tita Town. Kieran didn't think that the five riders from before could have the capabilities to kill someone stronger, hence the question!
Who killed two of the aid-givers outside Tita Town?
"Who did Mary's mother request aid from? Or should I say, how many people?" Kieran asked directly.