The weapon decides first — its scaling letters tell you which one or two attributes are worth your points — and skills, Pictos and Lumina all get chosen to fit that choice, not the other way round.
A build comes down to one choice that gets made early and rarely revisited: which one or two attributes the weapon you're carrying actually scales on. Everything downstream — where the next few attribute points go, which skills you keep equipped, which Pictos you slot — gets picked to fit that weapon, not the other way round. A rare weapon that scales on an attribute you've never touched loses to a common one that matches the points you've already spent.
Levelling up hands you three attribute points and one skill point, split across five stats: Vitality (Health), Might (Attack Power), Agility (Speed and Defense), Defense (Defense and Critical Rate), and Luck (Speed and Critical Rate). None of them is a trap on its own — Might raises damage no matter what you're holding — but a weapon rarely scales on Might alone, and it's the scaling attributes that turn a flat stat increase into a multiplier on your actual weapon damage.
If you've already sunk points into the wrong stat, a Recoat item resets attribute points from the upgrade menu at any Expedition Flag, at the cost of also clearing your equipped skills. It's a genuine reset, not a light nudge, so it's worth reading the next section before spending a level's worth of points on a whim.
Every weapon lists a letter grade — D up to S — next to the one or two attributes it scales on. That letter is the real stat to compare, more than the base Attack Power number sitting above it: a low grade barely moves with your investment, while a high grade turns points you've already put into that attribute into a much bigger damage swing. Two weapons with the same base power and different grades in the attribute you've built around are not close — the one with the better grade in your attribute wins, rarity notwithstanding.
This is why "just take the rarer weapon" is the wrong instinct here. A common weapon that scales on the attribute you've been feeding for ten levels can outperform a flashier drop that scales on Luck when your points are all in Might. Before committing to a weapon for the run, check what it scales on against what you've already invested — the full list, scaling included, is in the weapons database.
Attribute points make the weapon you're holding hit harder; levelling the weapon itself is a different budget entirely, paid in Chroma Catalyst rather than the Chroma you spend at merchants — the distinction, and what each material actually buys, is covered in the Parry, Chroma and Lumina guide. The short version for a build: a weapon's level and its attribute scaling stack, so a weapon you've levelled up rewards the same attribute investment even more than it did at level one, which is another reason to settle on a weapon before pouring Catalyst into it rather than after.
Skills belong to the character, not the weapon — Gustave, Lune, Maelle and Sciel each have their own tree, unlocked one skill at a time with the point you get every level. In battle you can only have six of them equipped, split across two tabs of three, so the tree you've unlocked is never the loadout you're actually using — the six you pick are.
That cap is the argument for specializing rather than collecting. A handful of skills built to work together and cycled every turn does more than six unrelated ones that each cover a different situation once. Lune is the clearest case of why: her skills both generate and consume elemental Stains on the battlefield, and a skill that reads "deals more damage if an Ice Stain is present" is only doing that extra work if another equipped skill is actually leaving Ice Stains behind. Committing to Fire as the main damage type and Ice as the stain that feeds it — rather than trying to keep Lightning and Earth in rotation too — is the kind of choice that makes six equipped skills add up to more than six separate ones. The same logic of "fewer moves, cycled harder" carries across characters even without Stains: a small set of skills you can afford to cast every turn on your Action Point budget beats a wide spread you can only half-afford. The complete skill list per character, with AP costs, is in the skills database.
Each character carries three Pictos, and each one grants both a stat bonus and a passive effect — which makes them the last piece to pick, because by the time you're choosing them you already know what the build needs more of. A Picto that pushes the attribute your weapon scales on reinforces the first decision in this guide; one that pushes AP or Speed reinforces the skill loop from the section above. Chasing whichever passive sounds strongest in isolation, without checking it against the weapon and skills already locked in, is how three good Pictos end up doing three unrelated things instead of one thing well.
Mastering a Picto by winning battles with it equipped turns its passive into a Lumina that any character can slot in later, decoupling the effect from the stat bonus — how that system works, and how many Lumina Points you actually have to spend, is its own subject and covered in full in the Parry, Chroma and Lumina guide. For picking Pictos in the moment, the question is the same one that's run through every step here: does this reinforce the attribute and the loop you've already committed to, or is it a different build wearing the same character. The full list of effects and where to find each one is in the Pictos database.