There are a couple of candidates who might fit your description:
The Father of Mortis, from the kinda weird Mortis arc of Star Wars: The Clone Wars. He's quite emphatic in his assertions that Light and Dark need each other in order to coexist:
Father: Too much dark or light would be the undoing of life as you understand it.
The Clone Wars Season 3 Episode 15: "Overlords"
In theory his two children, Son and Daughter, should also qualify, but they each lean in one direction or the other (this is actually kind of the point of Mortis), and the Son ultimately gives into the Dark Side entirely.
The Bendu, from season 3 of Star Wars Rebels, describes himself as being "in the middle" of the Light and Dark sides:
Bendu: Jedi and Sith wield the Ashla and Bogan, the light and the dark. I'm the one in the middle, the Bendu.
Star Wars Rebels Season 3 Episode 1: "Steps Into Shadow"
Despite his claims, we know little about him and his philosophies, so we don't know how close he actually comes to your "grey" definition.
That both of these characters are vaguely-cosmic entities is probably not a coincidence.
Honourable mention goes to Quinlan Vos, a Jedi Master who learns the ways of the Dark Side in the novel Dark Disciple. He tries to balance the Light and the Dark but, without wishing to spoil too much of the novel, fails spectacularly.
You might also consider Asajj Ventress in the same novel, who teaches Vos to use the Dark Side without being consumed by it; but it's not clear that she really uses the Light Side either, so it's debatable whether she meets your criteria.
That being said, as I've mentioned on the site before, this trope is one that the Lucasfilm Story Group generally dislike, so you're not likely to see too much of this in any canon storytelling.
The most prominent Legends examples are the Je'daii Order from the Dawn of the Jedi comic series. One of the earliest Force-wielding organisations, they absolutely meet your criteria, believing that the ideal state of the Force is balance between Light (which they called Ashla) and Dark (Bogan):
The Light defined the Dark, as the Dark did the Light. When balance was not maintained, Tython reacted to the imbalance with severe storms and quakes. And so the travellers defined themselves, ever seeking a balance. They became the Je'daii, a Dai Bendu term meaning "mystic center."
Dawn of the Jedi 001 "Force Storm Part One"
This eventually fell apart, as depicted later in the series, but there are a handful of individuals later who arguably take up their mantle.
But for your definition, Kreia, from the video game Knights of the Old Republic 2: The Sith Lords is probably the closest. By the time of the game, she's both a former Jedi Master and a former Sith Lord, but has given up both sides of the conflict; you can ask her about this at one point, and she pretty much word-for-word quotes your definition:
Exile: Kreia, what are you - are you a Jedi, a Sith?
Kreia: Does it matter? Of course it does, such titles allow you to break the galaxy into light and dark, categorize it. Perhaps I am neither, and I hold both as what they are, pieces of a whole.
Whether or not she actually wields Dark or Light side powers (or both) is up to the player, but she's the only one who can effectively wield both in equal measure1, and she philosophically rejects the Light Side/Dark Side dichotomy. There's an interesting moral choice at one point in the game where Kreia chews you out regardless of which you pick:
If you pick the Light side option (giving money to a beggar), she says:
Kreia: By giving him something he has not earned, perhaps all you have helped him become is a target. Seeing another elevated often brings the eyes of others who suffer. And perhaps in the end, all you have wrought is more pain. And that is my lesson to you. Be careful of charity and kindness, lest you do more harm with open hands than with a clenched fist.
If you choose the Dark Side option (threaten to kill the beggar), she says:
Kreia: Cruelty leads to suffering. And when one suffers, it is the way of life to spread suffering. The suffering within builds, until its sound is all one hears. And when a kindness is offered, it is punished. And a greater darkness is served.
That being said, even Kreia doesn't really meet all of your criteria; rather than seeing the merit in both sides of the Force, she sees them as equally flawed, and her goal is to "achieve balance between the two" in the most reductive way possible.
1 In the game, your alignment score affects how easily characters can use Light or Dark side powers; the more Light-sided characters get a bonus for Light-sided powers and a penalty to Dark-sided powers, and vice versa. Unless you play the game very carefully, Kreia is the only Force-sensitive character who maintains a completely neutral alignment2, meaning she suffers no penalties for using powers from either side
2 Jolee Bindo, a character in the first Knights of the Old Republic game, comes close here. But despite professing to "see more grey than dark or light", in the end he turns out to be a Light-sider (if you choose the Dark side, he proclaims himself to be a Jedi and attacks you), foreshadowed by the fact that his alignment is ever-so-slightly Light-sided