Inhuman Royal Family is probably the most unique synergy of the game so far as it comes in two halves.
Medusa has her Living Strands generate up to 6 Fury Buffs when added to a team with Black Bolt who has his Provocation buffs become permanent instead of temporary.
If you team Medusa with Karnak her Heavy Attacks inflict up to 4 permanent Armor Breaks and Karnak becomes Focused 35% faster and his True Strikes last 35% longer.
Want more? If you team a Medusa 4-Star or higher with Karnak and Black Bolt you get Inhuman Royal Family where your team gets +25% Attack at the start of a fight!