The wizard is
Leonhard Euler
and his "union" is
the famous equation $e^{i\pi}+1=0$.
The two "helpers" are
the + and = symbols
and the five others are
the actual numbers involved.
Here are my best guesses at how the clues work. Some of them are not very confident guesses.
An unpredictable guy, the wizard's adopted son, who was a prominent & important figure in the court of an English warlord!
This must be $e$, named of course for the "wizard" Euler and "unpredictable" because irrational. (Either because the digits look random, though actually that isn't implied by irrationality, or because irrational people may be unpredictable.) In comments, OP indicates that the English warlord is in fact another mathematician. Well, pretty much any mathematican in the last 300 years will have made much use of $e$. Newton? (Co-discoverer of the calculus, from which $e$ emerges very naturally.) Taylor? (The exponential function is strongly associated with its nice simple Taylor series.) I hope it isn't Napier, pioneer of logarithms, because Napier was Scottish and not English. Nor, because he too was Scottish, Maxwell (nothing to do with logarithms but plenty of "E"s in his work for entirely different reasons.) Perhaps the word "warlord" is a hint, but I have completely failed to figure out how. (Lots of mathematicians have done military work; hard to see how to pick out a particular one.) ... Or maybe "from the same profession" doesn't mean the warlord was a mathematician. Perhaps e.g. it refers to a "profession of faith"; Euler was a Christian, as we know from the silly story about him and Diderot, so perhaps our English chap was notably religious. I dunno.
This was actually an imaginary guy & as per our wizard he came from ancient Greece.
Presumably this is $i$ ("imaginary"). I've no idea in what sense "per our wizard he came from ancient Greece": I think Euler consistently wrote $\sqrt{-1}$ explicitly rather than (as some occasionally do) $\iota$, and the ancient Greeks certainly didn't have complex numbers. The letter $i$ is, I guess, derived from Greek $\iota$, but again I don't think Euler used it.
Another crazy guy, from somewhere in Asia. It was believed that he came from some sort of spiritual realm.
It seems this must be $\pi$, but I don't understand how this comes from Asia. (Though some of the earliest numerical approximations to it are Chinese.) $\pi$ is, though, a transcendental number, which is close enough to "spiritual" for our purposes here.
This guy is a representative of his people as well as this union. He was an old & ancient fellow, and nobody knew where he came from, though some people believe he attained this current status in India.
This must be either 0 or 1. Probably 1, perhaps with a suggestion of "one man, one vote" ("representative of his people"). Our numerals are of Indian origin.
Another old guy from India, who had the look of an ancient Himalayan yogi & claimed to hold the truth about whole cosmos.
If the previous one is 1 then this must be 0 (which definitely comes from India; more specifically, from Brahmagupta in what we parochially call the seventh century). The last bit might fit better if this one is 1 ("all is one"...) but it looks to me as if the numbers are being listed in the order of their appearance in the usual form $e^{i\pi}+1=0$ of the formula, in which case this must be 0. I suppose some people have claimed that our whole world is some sort of illusion, which would make 0 kinda-sorta hold the truth about the whole cosmos.
In comments below, Silenus suggests
putting #4 and #5 the other way around, 0 being "representative" because both sides of the equation are 0 and 1 for the same "all is one" reason I mentioned above -- and also because the numeral 1 is thin, like a Himalayan yogi! This could well be right; I can't work out whether I find these reasons more or less persuasive than the fact that otherwise the numbers seem to be "in order of appearance".
Things I'm most conspicuously confused by:
Who's the "English warlord", of the same profession as Euler, whose "court" prominently includes $e$? In what sense is $i$ Greek or $\pi$ Asian? If 0 and 1 appear in that order, in what sense is 0 "representative of his people"? If in the other order, why does 0 claim "to hold the truth about whole cosmos"?
