Two rulers, and the argument between them
Traditionally Aquarius belongs to Saturn, the same planet that runs Capricorn. Modern practice hands it Uranus instead. The two readings pull in opposite directions and both are in circulation, which is worth knowing before trusting any Aquarius description.
Saturn makes Aquarius a sign of structure, rules held on principle and a reluctance to be moved by sentiment. Uranus makes it a sign of disruption and sudden reversal. Anyone who has met a few Aquarians will recognise both, and the honest position is that the sign's reputation is a composite of two different astrological claims rather than one observed type.
Where the four names came from
Gemini and Libra complete the air group, each standing 120 degrees from Aquarius. Aries and Sagittarius are fire, 60 degrees off either side. Both separations were ranked easy long before anyone applied them to romance, and the names are just whatever sits at those distances.
The chain runs from a distance to a label to a sign to a person, and the last step is the only one anybody cares about and the only one with nothing behind it.
Fixed air, which explains the reputation for stubbornness
Aquarius sits in the middle of its season, so it is a fixed sign, and the material it holds fixed is opinion. That produces the familiar shape: unusually open about what may be considered, unusually immovable once a conclusion is reached. Against mutable Gemini the tradition expects a mind that plays met by a mind that has decided. Against cardinal Libra it expects two people who both prefer the discussion to the settlement.
Those are descriptions of how the categories interlock, offered without evidence and taken up as though they were findings.
What is actually being compared in a real reading
A sun sign is a single line in a chart that contains many. Any comparison with weight reads both Moons for what each person needs privately, both Venus placements for how affection is offered and recognised, both Mars placements for the shape of a quarrel, and both ascendants for the first impression each gives, then measures the aspects running from one chart to the other. That measurement is synastry; the synastry calculator carries it out from two birth times.
Even then, no soulmate emerges. The word is not part of the technical vocabulary, no aspect indicates it, and astrologers who use it are borrowing from elsewhere.
The pairing itself is the better read
Whatever combination is actually on your mind has a page describing what it manages and what it strains. Aquarius and Taurus, a square between two fixed signs, explains rather more than Aquarius and Libra does, because it has to account for a genuine deadlock.
Then compare the charts if the birth times exist. It is a narrower exercise and the only defensible one.
Questions people ask about this
Which signs are traditionally named for Aquarius?
Gemini and Libra in air, Aries and Sagittarius in fire. The four fall out of the angles rather than any study of couples.
Is there a soulmate placement?
No. Nothing in a chart signifies the idea, and it entered astrological writing from outside.
What rules Aquarius?
Saturn traditionally, Uranus in modern practice. The two produce noticeably different descriptions of the sign.
What does a serious comparison examine?
Moon, Venus, Mars and ascendant in both charts, and the aspects between the two charts.