Buff / Extra (?)
Starlight Stream (Both Twins require «One-Handed Sword»)
Starbright Smasher (Both Twins require «One-Handed Sword» and «Sprint»)
Crescent Twister (Both Twins require «One-Handed Sword», «One-Handed Spear» or «One-Handed Rapier» AND «Sprint»)
Meteor Circular (Both Twins require «One-Handed Sword» and an «Unarmed» skill)
Ecliptic Paradox (Both Twins require «One-Handed Sword»)
David's Star Splash (Both Twins require «One-Handed Rapier»)
Vorpal Penetration (Both Twins require «One-Handed Rapier»)
Mercury Strike (Both Twins require «One-Handed Rapier»)
A skill with a simple, yet cryptic description: "gives bonuses based on the other Twin's stats, as well as other things." Being a rather circumstantial-based Extra Skill that relies of pure luck, for the 10,000 people in «Sword Art Online», there have only been 10 reported cases of «this weird skill» - all reported cases of identical twins (in theory). The one of the first passive skills «Stat Boost», which increases STR and AGI by 2% of the Twin's own STR and AGI (max is +100%), getting double the effect when the equipment is identical. «Proximity Link», another starter passive skill, increases STR and AGI by a maximum of 100% base STR and AGI, but decreases by 1% per 25 meters (can reach up to negative) away from the Twin. «Exp Share» lets each Twin gain experience equal the the experience gained by the other Twin. Some advanced (and near Mastered) «Skills» may include «Intertwining Insight», «Interdependent Senses», «Mirror Boost», and «Reflective Insinuation», which have been purposely excluded from detailed description because of a certain client's request. Also, «all Passive Skills» and «Skill Points» of one Twin have the same Flags with the opposite Twin.
This skill cannot be «removed», or «replaced». There are no publicly known ways to level up this skill, as it only seems to level up randomly by keeping the «Passive Skills» active - or it's just that none of my only known sources have been able to fully confirm this information. The «Skill Points» of this skill do not (seem) to reduce when «unequipped». Until the Twin/s gain their 4th Skill Slot, this skill is not counted as a «used slot».
Must Be an Identical Twin
Identical Twin must be in «Sword Art Online»
A skill that relies on the powers of the twin. There has been a lot of talks throughout Aincrad about what is an «Extra Skill», though, but, while it's easy to conclude that, the easily forgettable reminder about an «Extra Skill» is that it is a «Hidden Skill» merely serves to disprove this - simply because all twins in SAO got this skill. It would be a good idea if I would ask «Heathcliff» , for some useful insight about this sometime, but he's always busy.
"Ah... but it can be unequipped right...? I mean, isn't it worrisome that you'll have to move with each other synchronously just to not fall...?"
(Response: Yes, what made you think otherwise?)
"I saw a twin slashing down on a field boss with an insane amount of cooperation. Never I have seen such, even in my days as a front-liner."
- a guy from the «Army»
(Response: It's an inevitable outcome, really.)
"The skills «Proximity Link» and «Intertwining Insight», as well as some of their specific «active skills» actually help in simply furthering their compatibility. It's quite a sight really."
- Heathcliff, during the «34th Floor Boss Battle»
(Response: Wait, how did he know of those passive skills?! Is this... the power of the «Living SAO Encyclopedia»?! *shock* )
Yup, this is the pseudo-Extra Skills section, a section of skills that may or may not be considered as an «Extra Skill», which will be explored after around 2 chapters. Also, Tech 2 Skill Tree in progress for Chapter 15. I need to contact PlainStoryTeller for the next skill... lol. Also, obvious error is obvious, if you read the other fic related to this skill. This is plot-wise intentional.