You're in a belief that this one person is your soul mate, and will never stop until he/she is yours alone. You are mentally unstable, but you are still in control. Whether or not your love knows of your condition, you will protect him/her with your life. Basically, you kill for your love.

Yandere (ヤンデレ) is a portmanteau of two Japanese words yanderu (病んでる), meaning to be sick, and deredere (デレデレ), which is defined as strongly and deeply exhausted, infatuated, moonstruck, head-over-heels, or lovestruck, but in this case used for "lovestruck."

True Love

Starting at 3rd level, you choose another PC as your "true love". When you are at least 10 ft from him/her, you can use your reaction to subtract 1d4 from any attack roll made towards you or "love". The amount of dice scales with your sneak attack dice. You can also use a reaction to grant advantage to your love's roll. Your love can also grant you advantage by using their reaction.

Stalker's Stride

At level 3, you become proficient in Sleight of Hand and Deception. Through stalking your "love" on a daily basis, you became quick on your feet and hands. Increase all speeds you can use by 10. Can take a Use an Object action as a bonus action.

Maniacal Laughter

At level 9, you just so insane, that you start laughing maniacally on the battlefield. Anyone who can hear and see you look at them, must make a Wisdom saving throw against your intimidation. If anyone who fails, are feared. They must make the save again on their turn to snap out of the fear. Additionally, if your love is with you, you have advantage.

Once you use this feature, you can't use it again until you finish a long or short rest.

All Of Me, All For You

At level 13, if you are within 10 ft of your love, you can swap places with your love when they take damage. You take the damage in place of your love. Does not use a reaction.

If your love falls unconscious, you let out a blood curling scream. All creatures, if they can hear, must roll a CON save against 13 + Intimidation. Creatures that fail become deafened and take 2d12 psychic damage. The effect ends once your love regains hp. If your love dies, you repeat the 2d12 psychic damage and die. If you succeed the check, you take only half damage. This only occurs once a battle.

Endless Love

At level 17, if your love starts their turn and is 5 ft away, you can follow them when they move as a reaction. If they make an attack while you are 5 ft away from them, you can also attack with advantage using your reaction. If you followed your love with your reaction, you don't need your reaction to make a follow up attack. You both have advantage on attack.