Baccano! ?

Basically, mafia + immortals = Baccano!, but that might not make much sense, so let me explain. In 1711, a group of alchemists on board of a ship called the Advena Avis discover the secret to become immortal. However, they soon realize it is much too dangerous to stay together, as they are being "eaten" by there own companions. They decide to scatter themselves across the world, hoping to stay alive that way. Fast forward to 1930, where we meet various mafia families, a hilarious thieving duo, and a street gang, who (surprisingly) all happen to have some sort of connection with eachother. Several of these people unknowingly end up drinking the elixer of life, which grants them eternal life. Ofcourse, there's more to Baccano!, but if you find yourself intrigued by this small summary, then just watch or read it for yourself ;)

Ennis

Ennis is a homunculus created by one of the original "Immortals", Szilard Quates. She was created with the intent to have no thoughts or emotions of her own, and to just follow the orders given to her. However, this is all changed upon meeting Isaac Dian and Miria Harvent, a thieving duo, who encourage her and are the first friends she makes. She starts to have thoughts and emotions of her own after this, and even tells the newly created Immortals how to "eat" eachother, which ends up with Firo Prochainezo "eating" Szilard and gaining all of his knowledge. Firo and Ennis fall for eachother, and get married in the 1990s.