Every weapon has exactly one paired passive. If you want an evolution, you have to grow those two together — no other passive evolves that weapon.
The Pairing Is Fixed and Unique
No two weapons ever share a passive. Once you've decided to take a weapon, take its pair when it shows up too; if you grow one without the other, that weapon can never evolve. It's the only pairing rule you need to keep in mind when picking cards.
Wide Roster, Narrow Focus
The roster has 40 weapons, and each has two evolution tiers. But you can't get to all of them in one run: deepening two or three weapons together with their pairs is far stronger than keeping eight weapons shallow.
Watch Out for Zone Weapons
Weapons that leave a lasting zone on the ground (such as the Void Well or the Nanite Swarm's cloud) can only keep a limited number of zones on the field at once. When the limit is reached, the oldest zone is removed — so stacking lots of zone weapons makes them eat into each other.
Don't Mix Up Similar Weapons
Some weapons are very similar, and the difference between them is a single thing: one leaves a lasting zone and the other doesn't; one fires while you stand still and the other while you move. Read the card's description; don't pick by the icon.
Some weapons have a movement condition: they only fire while you're below or above a certain speed. If you think a weapon isn't working at all, try it standing still, then try it while running.