GFL2 has two separate doll-upgrade systems with similar names. The Neural Helix is a node grid that gives stats and Keys. Neural Fortification is the dupe-fed system that upgrades skills — the 'V0 to V6' you see in tier lists. This guide separates them and gives an investment order. Costs and mechanics are from the in-game Doll Enhancement screens, reworded.
Neural Helix — the node grid
Unlocks at Commander Level 15. A grid of 13 nodes bought with Sardis Gold and Transcription Conductors I–VI (about 37,000 Sardis Gold and 440 assorted Conductors to fully clear an Elite doll).
Enhancement nodes (2, 3, 5, 6, 8, 9) give flat Attack, Defense, Health and Health Boost.
Evolved nodes (4, 7, 10) each offer two Fixed Keys — you keep one per node, so three of a doll's six Fixed Keys come from here. Every second Evolved node also adds an Attack / Health / Defense Boost.
The last node hands a gold Common Key into your shared pool. Some dolls have a 14th node holding their Expansion Key.
The Neural Helix does not touch skills — it is purely stats and keys.
Neural Helix — unlock order
Rush the Evolved nodes (4, 7, 10) and any yellow nodes — those are the Fixed Keys and the Common / Expansion key, the parts that change how the doll plays.
The plain Enhancement stat nodes are a small, flat increase; fill them once your main team's keys are sorted.
Some nodes need the doll at a minimum level to unlock, so level her alongside.
Neural Fortification — V1 to V6
This is the only way to upgrade a doll's skills. Six levels; each needs 30 of that doll's Neural Archives, which come from pulling duplicates.
Each level adds a clause to one of her skills — an extra stack, a new effect, a removed restriction. Each doll page lists her six Fortification clauses (labelled Fortification · V1 to V6) under Skills.
The tier list rates dolls at V6 with the signature weapon. Most are functional at V0; the gap to their rated level is the Fortification clauses plus the weapon.
A few dolls get Neural Archives outside the gacha: Sabrina from the Battlelog Trading shop, Lotta from Neural Integration, Papasha and Balthilde from the Platoon shop — those four are realistically V6-able for free.
Reading the V6R1 shorthand
'V' is the Fortification level (vertebra) — V6 means you've pulled seven copies of the doll in total.
'R' is the signature weapon's Calibration level — R1 means two copies of that weapon. Calibration boosts the weapon's own skill, six times over, and needs a duplicate of the same weapon type and rarity.
So 'V6R1 Springfield' is max Fortification plus her weapon Radiance calibrated once.
Where to spend first
Neural Helix Evolved nodes for your main five dolls — cheap, and the Fixed Keys are a real power gain.
Fortification only as far as each doll's first skill-changing clause (shown in her Recommended build box) — chasing a full V6 on a free-to-play account is a copies problem, not a materials one.
Enhancement stat nodes and Remolding Core last — they're the flat-percentage filler once the keys and skills are in place.