Core Linking is the process of using one of your unit as a "core" to "link" with another unit, increasing the latter's Stats, but also its Sortie Cost.
Important Note[]
Link Breaker 
is scheduled to be removed. Until the removal takes place, Self-Learning Algorithm 
is given every month, thus removing link and changing full link bonus are free actions and require nothing, as long as you have received the Self-Learning Algorithm from Mailbox.
Core Linking[]
In order to Core Link, the unit must have a free Core Link Slot (Unlocked at level 10/30/50/70/90 to a maximum of 5). Click on an empty slot then either use a Proxy Core (S-Rank Core for S-Rank or lower Units, SS -Rank Core for SS-Rank Units) or select the unit being used as the "core".
Each Unit have a specific compatibility with the main unit (depending on Rarity, Class and Type), except duplicates, which have 100% compatibility. The unit used as Core Link will be lost permanently.
Alternatively, you can use a Proxy Core as the "core" instead of units. These Special fodders transform into the duplicate of the main Unit, creating a link with 100% compatibility.
In order to remove a Link, you need to use a Link Breaker. As mentioned in Important Note, removing a link from a unit is a free action.
NOTE: Although you can use any Units for Core Linking, but it’s ill-advised to use anything other than a duplicate or a Proxy Core, because using a non-duplicate will give a lower stat boost, and will not count towards the unit’s Full Link Bonus, which needs maximum Compatibility.
NOTE 2: Removing a Link will destroy the linked core/unit.
NOTE 3: The compatibility only depend on the Unit being used as the "core". Level and Upgrades do not affect Link Compatibility.
Link Bonus[]
The Stats that is raised by Link Compatibility. Each 100% Compatibility increase Unit's Stats as follows:
- HP +20%
- ATK +20%
- Differs Between Units
- EXP +4%
List of possible Stats for the 3rd Bonus:
- Skill Multiplier +2%
- HP +5%
- DEF +3%
- SPD +0.02
- CRIT +2%
- EVA +2%
- ACC +7%
NOTE: Skill Multiplier is additive to Active Skills' Original multiplier. For example, if the skill's original multiplier is 245% and the bonus multiplier is 15%, multiplier of that skill become 260%.
Full Link Bonus[]
Unlocked when a unit reaches 500% Compatibility (all 5 Cores are Duplicates/Proxy Cores). Unlike Link Bonus, you may only choose to use one of the 5 given Bonuses at a time. Changing from one Bonus to another consumes a Link Breaker. As mentioned in Important Note, changing your Full Link bonus does not consume any item.
Possible Full Link Bonuses:
- Sortie Cost -XX%
- Skill Multiplier +XX%
- Differs Between Units
- Differs Between Units
- SPD +XX
Possible generic options for the 3rd and 4th Full Link Bonus:
- Range +1
- CRIT +20%
- Buff/Debuff Level+2
- ACC +75%
- HP +20%
- EVA +20%
NOTE: Skill Multiplier is additive to Active Skills' Original multiplier. For example, if the skill's original multiplier is 245% and the bonus multiplier is 15%, multiplier of that skill become 260%.
NOTE 2: Buff/Debuff Level+2 only affects Effects listed in Skill Effects in Unit Details and not multiplier.
NOTE 3: Certain units have unique Link Bonuses available only to them. An example for this is Frigga who gets 50% HP instead of 20% and Cyclops Princess/Goltarion who don't have max HP bonus.
QNA[]
- Is it worth linking unit that's not 100%?
Unless you're absolutely sure what you're doing (Since you're asking this, I assume no), then no.
- Do I need to level up the spare units that I am going to use as a core link?
No.
- Who should I focus my SS Proxy Core on?
Invincible Dragon, Labiata, and Lemonade Alpha. After that it's up to you.
- Which link bonus I should choose for <Insert unit here>?
The one you need. Do you need more base damage? Pick the skill multiplier. More stat? Pick stat. Faster unit? Pick SPD. Sortie cost down? Pick sortie reduction. Higher buff/debuff power? Take buff/debuff level. Don't know which to pick? Don't pick any until you do. You will soon find out as you play the game.
- Is it worth linking <Insert unit here>?
Think, player. Does linking that unit worth the increased cost of deploying that unit? What does linking adds up to a unit? Read Core Linking to find out.
Alternatively, simply keep a full link and a no link copy of said unit. Because you can (with exception of Invincible Dragon, Labiata, and Lemonade Alpha).
- I cannot link my unit. The error pop-up has yellow/orange text under white text.
That unit is in a squad preset. Remove them from the preset.
- If I rank up a unit to SS, do I have to use the SS Proxy Core to link them?
No.