In function, there are multiple aspects to the Foxkit Fang. By default, it grants Resistance +3. In addition to this; if Selkie has higher Res than her opponent and they are using a melee weapon, she gains a bonus to all of her stats through combat equal to 50% of the difference between their Res stats; with a maximum bonus of +8. It also provides the standard beast transformation effect of Attack +2 if she starts the turn adjacent to beast or dragon allies; or alone, with the cavalry effect of countering follow-up attacks and inflicting an in-combat Attack & Defense debuff when she initiates.
Grants Res+3. At start of combat, if unit's Res > foe's Res and if foe uses sword, lance, axe, dragonstone, or beast damage, grants bonus to Atk/Spd/Def/Res = 50% of difference between stats. (Maximum bonus of +8 to each stat.) At start of turn, if unit is adjacent to only beast or dragon allies or if unit is not adjacent to any ally, unit transforms (otherwise, unit reverts). If unit transforms, grants Atk+2, and if unit initiates combat, inflicts Atk/Def-4 on foe during combat and foe cannot make a follow-up attack.