Seekers are Templars.
Seekers are typically chosen from the elite ranks of the templars and used to train at isolated fortresses such as Therinfal Redoubt in Ferelden. In their roles as overseers of the Templar Order and the right hand of the Divine, they serve as the Chantry's investigators and are given considerable latitude to function outside the law.The leadership hierarchy of the Seekers runs parallel to that of the templars, although a senior Seeker outranks any templar.
They use the rite of tranquility and reverse it with the use of a spirit to gain their powers and be able to use similar Templar Powers without Lyrium (except more special snowflake abilities). Their gear is much darker colouring than normal Templar gear but they are still templars. Cassandra is the right hand of the Divine and a Seeker, her armour is probably "special" like the Lord Seeker so they have to stand out.
Also, could be just time constraint and cut content, which is most likely the cause.