The concept of a "Lebensmensch" plays a dominant role in Thomas Bernhard's works. At Wikipedia one reads that
»Lebensmensch [is] a predominantly Austrian term […] which refers to
the most important person in one's life«
This is a somehow typical usage of the word by Thomas Bernhard (from Wittgenstein's Nephew, translated by DeepL):
»But in truth, even without Paul, I would not have been alone on the
Baumgartnerhöhe in these days and weeks and months, for I had my
[Lebensmensch], the decisive one for me in Vienna after the death of my
grandfather, my [Lebensmensch], to whom I not only owe a great deal,
but, frankly, since the moment she appeared at my side over thirty
years ago, more or less everything. […] The initiates know what
everything is hidden behind this word Lebensmensch, from and through
which I have drawn my strength and survival over thirty years, from
nothing else, that is the truth.«
For the sake of comparison, here the translation by Google:
»But in truth, I would not have been alone at [Baumgartnerhöhe] these days and weeks and months without Paul, because I had my [Lebensmensch], the one who was decisive for me after the death of my grandfather in Vienna, my [Lebensmensch], who I not only much, but, frankly, since the moment she appeared by my side over thirty years ago, I owe more or less everything. […] The initiates know what is hidden behind this word "living person", from and from which I have obtained my strength and my survival over and over again, from nothing else, that is the truth.«
I wonder if there is such a beautiful one-word-term for the most important person in one's life (except for oneself) in the English language.
Best Answer
Generally, the term you're describing is called a term of endearment, or a way to affectionately refer to a person you hold dear. There are many of them, from shortened names (Bess for Elizabeth) to animals (bunny) or sweet things (honey, sugar) to adjectives (beautiful, gorgeous).
The most important person is more specific. Your criterion suggests there is only one who is above all others. If you're trying to find a companion term (pun intended), you have a few options:
Soulmate or soul mate, as mentioned by another answer, suggests an ideal fit between two people who connect not merely physically but soul-deep. Oxford English Dictionary, "soulmate, n.":
According to Merriam-Webster, Samuel Taylor Coleridge is said to have invented the word in an 1822 letter to a young lady, where he says:
If you could go for a phrase, the love of my life is a common collocation for someone most dear to someone else. Even beyond the Queen song, the phrase has had wide currency from the 19th century onward. For example, here is an excerpt from "Only A Flirtation." Salem Observer, vol. XL, no. 10, 8 Mar. 1862, p. 1, as found in ReadEx's America's Historical Newspapers database:
Similar phrases extend back to Middle English, including lef lif (dear life) from the Middle English Sir Orfeo:
Finally, English has a habit of borrowing terms of endearment from other languages. Ma chérie and mon amie both come from French; inamorata comes from Italian; there are many nonce uses based on users' language backgrounds. Lebensmensch is a coinage from the author Thomas Bernhard, and the book Thomas Bernhard by Gitta Honegger uses the word as-is when explaining it:
Why not just use lebensmensch, treating the sentiment behind it as effectively untranslatable? You can also translate it literally, accepting that it's a metaphor, as life support or (as just suggested in a comment by AndrewL64) lifeline?