Seele is the committee lead by Lorenz Kiel, (the guy with the visor on his eyes) that controls Nerv (well, they think they do). There are 12 ppl in Seele all together so that’s why ppl refer seele in the 3rd person as it is a group, not an individual.
Also, in the first few episodes they refer to Seele as the committee (the group of 5 holographic ppl that sit around the desk with Gendo) the committee are a few members of seele that deal with the not so important matters of nerv – so you know things are bad when the whole group of seele show up, these are seen as the black holographic monoliths, there’s a 2001: a space odyssey reference there ^^
A. Publicly Released Information
B. Generally Recognized Information
A secret society that puppets Nerv from the shadows towards the fulfillment of the Human Instrumentality Project. Nerv's financial backing comes almost entirely from them.
C. Confidential Information
An esoteric secret society born during the medieval Dark Ages and, originally, a religious order. Slowly but surely, they extended their influence, until at last they crushed the final forces of opposition in the mid-20th century and became a latent power with covert rule over human society.
D. In-Depth Information
Upon acquiring the Secret Dead Sea Scrolls, Seele turned once again to a creed that, up until then, they had considered naught but the pipe dreams of their ancestors. By putting the miracle of divinity, in the form of prophecy, within their sights, they restored the faith. Their dogma is the Path to Adam Kadmon -- that is, the approaching of a divinity both ageless and undying
Also, in the ep24', there are 15 monoliths on the lake, which I've argued wasn't a visual blooper or retcon error. The actual organization is larger than 12 (or 15), those are just the leaders. The Human Instrumentality Committee consists of a subset of these 12 (5 of them including Keel) and includes Gendo.