Blood Is Thicker.

Riku x Seki.

Riku the high school "bad boy" breaking girls hearts left and right that is till he met Seki.

Seki the high school "good girl" keeping quite and pretty un noticed that is till she met Riku.

When they got marks on there necks that can't an wont go away leading you to your soulmate Riku and Seki share pale purple.

if someone cheats on the other the mark will turn white and you will lose all ability to love anyone.