At first, Bond plans to leave MI6 to be with her but after she's unveiled as a double-agent and killed, he turns into the cold, brutal and emotionally detached spy we all know so well. The way in which Vesper develops Bond is also remarkable. There's so much nuance and pathos to her and she's a character viewers will care about hugely, which makes the fact that she has possibly the saddest story of any Bond character - after being blackmailed into betraying her country and the man she loves, she kills herself to protect Bond - all the more crushing.
Secondly, and even more importantly, she's a generally great character. Firstly, she's a sublime, charismatic love interest who's very well-played by Eva Green and has flawless chemistry with Bond - in fact, their relationship is probably the best in the whole franchise. There are two main reasons why Vesper Lynd is the GOAT when it comes to Bond Girls.