In most anime, enemies stay enemies. After all, anime villains are capable of some amazingly terrible things, and it would look pretty questionable if the heroes let them off the hook. However, occasionally characters who initially despised each other move past their feelings of hate, and start trying to form new loving connections. Whether the spat was caused by a mutual feeling of annoyance, or by straight up death match, romances that grow out of hatred can be surprisingly heartwarming.
When Shouko Nishimiya and Shouya Ishida of A Silent Voice were in elementary school, Shouya bullied Shouko relentlessly because she was deaf. Though Shouko tried to build a friendly relationship, her efforts failed, and the two parted as enemies. When they meet again as teenagers, Shouya feels terrible about how he treated Shouko.
Slowly, the two begin to develop a more positive, romantic relationship with no bullying involved.
Kaname Ohgi and Villetta Nu start off Code Geass as mortal enemies. The fictional nation of Britannia has taken over Japan, and Villetta is a loyal Brittainian who sees the Japanese (called Elevens) as scum. Meanwhile, Ohgi is part of a Japanese terror group that is trying to take the country back.
Despite their vastly different political stances, and Villetta's unapologetic bigotry, the two eventually begin to recognize their shared humanity and end up falling in love.
For many Dragon Ball Z fans, Vegeta and Bulma seem like a supremely unlikely pair. The alien Vegeta starts off the series hating humans, and wants nothing more than to defeat his fellow Saiyan Goku. After being forced to work with Goku and his friends to defeat Frieza, his personality begins to soften, and he starts to develop relationships with humans, including Bulma.
Bulma isn't taking any of Vegeta's sh*t, but once he stops being so villainous, she's able to see the good in him. The two eventually get married and have two children, Bra and Trunks.
The protagonists of Maoyuu Maou Yuusha are the Demon King and the Hero, also known as Maou and Yuusha. At first, Yuusha wants to kill Maou, but when he actually meets her, he's struck by her beauty and intelligence.
He gives up on the idea of slaying the Demon King when she persuades him to help enact a plan to unite demons and humans, which will end the war that has threatened them both.