Why is Seigi's tattoo described as "triggerless"?
The manga refers to it as 無鍵 or "keyless." The anime uses the same term (see the Ep. 3 summary).

As for what these keys refer to, 


    There are actually 4 Void Makers and they are the keys to the Tattoo Ruins. It seems that every Void Maker needs a host to become a "key." So apparently Seigi's Void Maker had no compatible host prior to him,  thus making it "keyless." 


His trigger however seems to be blood.