Klaus and Caroline - The Vampire Diaries/The Originals
Klaus originally started off as the "big bad" in The Vampire Diaries making a relationship between him and Caroline the furthest thing from our minds. As the show went on, Klaus fell in love with her and Caroline realized that she cared for him too. They sleep together in the 100th episode, but unfortunately, nothing more comes from it on the show. However, Caroline is currently appearing on The Originals and the two have reconciled their friendship and seem to be on the verge of something more.
Clay and Justin - 13 Reasons Why
In Season 1, Clay and Justin were the farthest thing from friends. They hated each other and Justin actually put Clay in quite a few dangerous situations. In Season 2, Clay rescues Justin from living on the streets, invites him into his home, and attempts to help him put his life back together. They end up forming a friendship that is obviously very important to the both of them. After realizing just how complicated Justin's parental situation is, Clay's parents decide to adopt him making the two officially brothers.
Clarke and Lexa - The 100
To say that Clarke and Lexa started out as enemies is an understatement. They were quite literally the leaders on opposite sides of a war. When both of their people's lives were in jeopardy, they formed an alliance and Clarke showed Lexa a new way of life. Lexa immediately fell in love with Clarke and after getting over her betrayal at Mount Weather, Clarke reciprocated her feelings. Despite its tragic end, Clarke and Lexa were able to reunite one last time when Lexa saved her in the City of Light.
Hook and Emma - Once Upon a Time
The only thing Hook cared about when he met Emma was getting revenge. Emma only saw him as someone who couldn't be trusted and there were a decent amount of betrayals and sword fights between them. Regardless of their rocky start, circumstances forced them to work together and Hook ended up falling for Emma pretty hard. It took a while to get there, but Emma ended up falling just as deeply in love with him, and they are married with a child in the final season.
Cheryl and Toni - Riverdale
Cheryl wanted nothing to do with Toni went they first met. It wasn't until Toni offered Cheryl friendship during a particularly difficult time that something began to take shape. Cheryl never had a lot of people who truly cared about her, and Toni quickly became the person Cheryl needed. When Cheryl was trapped in conversion therapy Toni broke in to save her, and Cheryl was quick to jump into action the moment Toni was held captive by Penny Peabody. Their romance is heart-warming, and there's no doubt that these two would do anything for each other.
Lucy and Flynn - Timeless
Lucy and Flynn were nowhere near friends at the beginning of the series. They found themselves on opposite sides during multiple occasions and were against each other on almost everything. But like they say, there's a fine line between love and hate. Despite their differences, Lucy and Flynn grew increasingly closer and have started to lean on each other heavily. It feels safe to say that a friendship and a deep regard has formed between them and they've come a long way from where they began.