Board Thread:Questions and Answers/@comment-32940936-20170824203741/@comment-1458758-20170824212957

BE WARNED, SPOILERS AHEAD!

Familiars and Summons are effectively interchangeable. There are some exceptions, in the case of say Angel's Caliburn, who is a familiar, but is always on Angel's person, or Yukio's Naiads, who don't have a formal contract with Yukio, but are manifested from his bullets.

Kuro is Rin's familiar in name only really. They don't have a formal contract, so Rin doesn't provide Kuro power and he can't summon Kuro to him. Their partnership was really just Yukio BSing the higher ups so they wouldn't exorcise Kuro.

In the case of other familiars, it's covered in one of the volume releases, that the circles are really just a way to invoke the demon into appearing. High class tamers who have good relationships with their familiar don't have any need for circles. You can see this change in Izumo over the course of the Illuminati Arc, and you'll notice that Shima doesnt use a circle to call Yamantaka and Lightning barely does anything to call on demons.

In the case of Shura, she wields a Demon sword, and it's noted (Again in volume releases) that those who have demon swords should probably have both the Knight and Tamer meisters to be safe, so she learned that, and since she has an affinity for them, can summon Nagas. Her tattoo is actually the contract seal her family has with the demon, Hachirotaro Okami. How she actually summons the nagas I don't think has been seen, so whether or not she uses a circle is unclear, though in the anime they were shown appearing from talismens.

I hope this answers all your questions, feel free to message me if you have any more.