Saranggwa Pyeonggangui Ondal!

Sarang Park has only one mission in life: to save her cousin, Pyeonggang, from a life-threatening illness. The only way to do so is to find Pyeonggang’s “Ondal” -- the person Pyeonggang has been fated to be with since her previous life. But on her journey to connect her cousin with her other half, Sarang just might get tangled up in a romance of her own. Source: Webtoon