Yandere:
"Yandere" is a term for a character who at first is very sweet, loving, innocent, and overly affectionate like a normal deredere character, but over time begins exhibiting signs of being mentally sick as a result of having a too strong love. Yandere is a sickness born from too much love. They are the kind of character who will act all deredere and lovey-dovey when their relationship with their romantic partner is going well, but when something triggers jealousy or possessiveness they act impulsively in a way that makes them seem mentally sick.
Their love is expressed as a pathological obsession towards their love interest, and their infatuation and devotion to their love interest is so strong that it causes them to gradually develop an unstable mental state and become "sick". They become so attached to their love interest that it's impossible to let go and they may begin doing immoral things to improve their relationship, such as stalking their love interest or even using violence as a last resort towards anyone who "threatens" their relationship. They entrust themselves to their love interest, trusting in their love interest's kindness. They probe the deepest recesses of their love interest's heart to more perfectly form their greatest self within their love interest.