As for the leader of the Blood Clan's Secret Speakers,
There is not much information available in the outside world; they only know it's a female member of the Blood Clan.
Leaders of other factions serve at most a few decades, before a new leader takes their place, causing some personnel changes or reforms.
This is not the case with the Secret Speakers, they have not changed their leader in hundreds of years.
Because of her long-term leadership, the direction of development is relatively unified, allowing the initially small faction of the 'Secret Speakers' to gradually enter the public's view.
They began to become an important intelligence department within the association.
Slyther's letter does not explicitly say so, but by piecing it together,
we can infer the source of the information related to the Blood Clan.
The individual who could bring benefits to him and Slyther is this centuries-old leader.